Since Intel Corporation designed and manufactured a 4-bit microprocessor chip in 1971, CPUs have evolved from Intel 4004, 80286, 80386, 80486 to Pentium and Pentium II, and from 4-bit, 8-bit, 16 bit, 32-bit to 64 bit in over 20 years; The main frequency has increased from a few megabytes to over 400MHz today, approaching GHz; The number of transistors integrated in CPU chips has jumped from 2000 to over 5 million; The scale of semiconductor manufacturing technology has increased from SSI, MSI, LSI, VLSI to ULSI. The number of encapsulated input/output (I/O) pins has gradually increased from tens to hundreds, and may reach 2000 by the beginning of the next century. All of this is truly a revolutionary change.

For CPUs, readers are already familiar with 286, 386, 486, Pentium, Pentium II, Celeron, K6, K6-2... I believe you can list a long list like a treasure trove. But when it comes to the packaging of CPUs and other large-scale integrated circuits, not many people know about it. The so-called packaging refers to the outer shell used for installing semiconductor integrated circuit chips. It not only plays a role in placing, fixing, sealing, protecting the chip and enhancing its thermal performance, but also serves as a bridge between the internal world of the chip and the external circuit - the contacts on the chip are connected to the pins of the packaging shell with wires, which are then connected to other devices through wires on the printed board. Therefore, packaging plays an important role in both CPUs and other LSI integrated circuits. The emergence of new generation CPUs is often accompanied by the use of new packaging forms

The packaging technology of chips has undergone several generations of changes, from DIP, QFP, PGA, BGA to CSP and then to MCM. The technical indicators have been advanced from generation to generation, including the ratio of chip area to packaging area becoming closer to 1, the applicable frequency becoming higher and higher, the temperature resistance performance becoming better and better, the number of pins increasing, the pin spacing decreasing, the weight decreasing, the reliability improving, and the use becoming more convenient.

The specific packaging form will be explained in detail below.

1、 DIP packaging

The popular option in the 1970s was Dual In Line Package (DIP). The DIP packaging structure has the following characteristics:

1. Suitable for PCB perforation installation;

2. It is easier to route PCB than TO type packaging;

3. Easy to operate.

The DIP packaging structure forms include: multi-layer ceramic DIP, single-layer ceramic DIP, and lead frame DIP (including glass ceramic sealing, plastic packaging structure, and ceramic low melting glass packaging)

The important indicator for measuring the advancement of a chip packaging technology is the ratio of chip area to packaging area, and the closer this ratio is to 1, the better. Taking a CPU with 40 I/O pins encapsulated in a plastic dual in-line package (PDIP) as an example, its chip area/package area=3 × 3/15.24 × 50=1:86, which is far from 1. It is not difficult to see that this packaging size is much larger than the chip, indicating low packaging efficiency and occupying a lot of effective installation area.

During this period, Intel's CPUs such as 8086 and 80286 were packaged in PDIP.

2、 Chip carrier packaging

In the 1980s, chip carrier packaging emerged, including ceramic Leadless Ceramic Chip Carrier (LCCC), plastic Leaded Chip Carrier (PLCC), Small Outline Package (SOP), and Plastic Quad Flat Package (PQFP)

Taking a CPU packaged in QFP with 208 I/O pins and a center distance of 0.5mm between solder pads as an example, with an external size of 28 × 28mm and a chip size of 10 × 10mm, the chip area/package area=10 × 10/28 × 28=1:7.8, indicating that QFP significantly reduces the package size compared to DIP. The characteristics of QFP are:

1. Suitable for using SMT surface mount technology to install wiring on PCBs;

2. The package has small external dimensions and reduced parasitic parameters, making it suitable for high-frequency applications;

3. Easy to operate;

4. High reliability.

During this period, Intel's CPUs, such as the Intel 80386, used plastic quad pin flat packaging PQFP.

3、 BGA packaging

In the 1990s, with the advancement of integration technology, equipment improvement, and the use of deep sub micron technology, LSI, VLSI, and ULSI emerged successively. The integration degree of silicon single chips continued to increase, and the requirements for integrated circuit packaging became more stringent. The number of I/O pins increased sharply, and power consumption also increased accordingly. To meet the needs of development, a new variety, Ball Grid Array Package (BGA), has been added on the basis of the original packaging varieties.

BGA has become the best choice for high-density, high-performance, multifunctional, and high I/O pin packaging of VLSI chips such as CPUs and north-south bridges since its emergence. Its characteristics include:

Although the number of I/O pins has increased, the pin spacing is much larger than QFP, thereby improving the assembly yield;

Although its power consumption increases, BGA can be soldered using the controllable collapse chip method, abbreviated as C4 soldering, which can improve its electrical and thermal performance:

3. The thickness is reduced by more than half compared to QFP, and the weight is reduced by more than 3/4;

4. The parasitic parameters are reduced, the signal transmission delay is small, and the frequency of use is greatly increased;

5. Assembly can use coplanar welding, with high reliability;

6. BGA packaging still occupies too much substrate area, just like QFP and PGA;

Intel Corporation uses ceramic pin grid array packaging CPGA and ceramic ball grid array packaging CBGA for CPU chips with high integration (over 3 million transistors per chip) and high power consumption, such as Pentium, Pentium Pro, and Pentium II. Micro exhaust fans are installed on the housing to dissipate heat, thereby achieving stable and reliable circuit operation.

4、 New packaging technologies for the future

BGA packaging is more advanced than QFP and better than PGA, but its chip area/packaging area ratio is still very low.

Tessera has made improvements based on BGA and developed another packaging technology called μ BGA. With a center to center distance of 0.5mm solder pads and a chip area to package area ratio of 1:4, it has taken a big step forward from BGA.

In September 1994, Mitsubishi Electric of Japan developed a packaging structure with a chip area/package area ratio of 1:1.1, which had a slightly larger external dimensions than bare chips. That is to say, the size of a single IC chip determines its packaging size, giving rise to a new form of packaging called Chip Size Package (CSP). CSP packaging has the following characteristics:

1. Satisfies the increasing demand for LSI chip lead out pins;

2. Solved the problem of IC bare chips being unable to perform AC parameter testing and aging screening;

3. Is the packaging area reduced to BGA? To 1/10, the delay time is reduced to extremely short.

Someone once thought that when a single chip cannot yet achieve the integration level of multiple chips, it is possible to assemble high integration, high-performance, and high reliability CSP chips (using LSI or IC) and application specific integrated circuit chips (ASIC) on high-density multi-layer interconnect substrates using surface mount technology (SMT) to form a variety of electronic components, subsystems, or systems. This idea gave rise to the Multi Chip Model (MCM). It will have a significant impact on modern fields such as computers, automation, and communication. The characteristics of MCM include:

1. The encapsulation delay time is reduced, making it easy to achieve high-speed component development;

2. Reduce the size and weight of the entire machine/component packaging, and generally reduce the volume?, Reduce weight by 1/3;

3. The reliability has been greatly improved.

With the advancement of LSI design technology and processes, as well as the use of deep sub micron technology and miniaturization to reduce chip size, people have developed the idea of assembling multiple LSI chips into a precision multi-layer wiring shell to form MCM products. Furthermore, another idea emerged: integrating circuits from multiple chips onto a large wafer, leading to a shift in packaging from a single small chip level to a wafer level, which in turn led to the development of System On Chip (SOC) and PC On Chip (PCOC) chips.

With the advancement of CPUs and other ULSI circuits, the packaging forms of integrated circuits will also have corresponding developments, and the progress of packaging forms will in turn promote the advancement of chip technology.

Basic concepts of PCB design

1. The concept of 'Layer'

Similar to the concept of "layers" introduced in word processing or many other software to achieve nesting and synthesis of graphics, text, colors, etc., Protel's "layers" are not virtual, but the actual copper foil layers of the printed board material itself. Nowadays, due to the dense installation of electronic circuit components. Due to special requirements such as anti-interference and wiring, some newer electronic products use printed boards that not only have two sides for wiring, but also have a specially processed interlayer copper foil in the middle of the board. For example, the printed board materials used in current computer motherboards are mostly four or more layers. These layers are mostly used for power wiring layers with relatively simple wiring arrangements (such as Ground Dever and Power Dever in software) due to their relatively difficult processing, and are often wired using large-area filling methods (such as ExternaI P1a11e and Fill in software). The surface layer of the upper and lower positions needs to be connected to the middle layers using the so-called "Via" mentioned in the software. With the above explanation, it is not difficult to understand the concepts of "multi-layer solder pads" and "wiring layer settings".

For example, many people complete the wiring and only realize that many of the wiring terminals do not have solder pads when they print them out. In fact, this is because they ignored the concept of "layer" when adding device libraries and did not define the characteristics of the solder pads they drew and packaged as "Mulii Layer". It should be noted that once the number of layers of the printed board is selected, it is necessary to close those unused layers to avoid causing trouble and taking unnecessary detours.

2. Via

To connect the lines between each layer, drill a common hole at the junction of the wires that need to be connected on each layer, which is called a via. In terms of technology, a layer of metal is chemically deposited on the cylindrical surface of the through-hole wall to connect the copper foils that need to be connected in the middle layers. The upper and lower sides of the through-hole are made into ordinary pad shapes, which can be directly connected to the lines on the upper and lower sides or not connected. Generally speaking, there are the following principles for handling through holes when designing circuits: (1) try to minimize the use of through holes. Once a through hole is selected, it is necessary to handle the gap between it and surrounding entities, especially the gap between the lines in the middle layers that are not connected to the through holes and the through holes that are easily overlooked. If it is automatic wiring, you can select the "on" item in the "Via Minimiz8tion" submenu to automatically solve it. (2) The larger the required current carrying capacity, the larger the required via size, such as the vias used for connecting the power layer and the geological layer with other layers.

3. Screen printing layer (Overlay)

For the convenience of circuit installation and maintenance, the required logo patterns and text codes are printed on the upper and lower surfaces of the printed board, such as component numbers and nominal values, component outline shapes and manufacturer logos, production dates, etc. Many beginners only pay attention to the neat and beautiful placement of text symbols when designing silk screen layers, ignoring the actual PCB effect produced. On the printed circuit boards they designed, characters are either blocked by components, or they invade the soldering area and are erased. Some even label adjacent components, which will cause great inconvenience to assembly and maintenance. The correct principle for arranging characters in the silk screen layer is: "no ambiguity, take advantage of every opportunity, and be beautiful and generous".

4. The particularity of SMD

The Protel package library contains a large number of SMD packages, which are surface mount devices. The biggest feature of these devices, apart from their small size, is the single-sided distributed element pin holes. Therefore, when selecting such devices, it is necessary to define the location of the device to avoid "missing pins". In addition, the textual annotations related to these components can only be placed along the surface where the component is located.

5. Grid shaped External Plane and Fill Zone

As their names suggest, the network like filling area is a process of processing large areas of copper foil into a mesh, while the filling area only retains the copper foil intact. Beginners often cannot see the difference between the two on the computer during the design process. In essence, as long as you enlarge the drawing, it will be clear at a glance. It is precisely because it is not easy to distinguish between the two in daily life that the distinction between the two is not paid attention to when using them. It should be emphasized that the former has a strong suppression effect on high-frequency interference in circuit characteristics, and is suitable for places that require large area filling, especially when using certain areas as shielding areas, partition areas, or high current power lines. The latter is often used in areas such as general wire ends or turning points that require small area filling.

6. Pad

Solder pads are the most commonly encountered and important concept in PCB design, but beginners tend to overlook their selection and correction, and use circular solder pads indiscriminately in design. When selecting the type of solder pad for a component, factors such as the shape, size, layout, vibration and heating conditions, and force direction of the component should be comprehensively considered. Protel provides a range of solder pads of different sizes and shapes in the packaging library, such as circular, square, octagonal, round square, and positioning solder pads, but sometimes this is not enough and requires self editing. For example, for pads that generate heat, are subjected to high forces, and have high currents, they can be designed in a "teardrop shape" by themselves. In the design of pin pads for row output transformers on familiar color TV PCBs, many manufacturers adopt this form. Generally speaking, when editing solder pads on your own, in addition to the above, you should also consider the following principles:

(1) When the length of the shape is inconsistent, the difference between the width of the connection and the specific side length of the solder pad should not be too large;

(2) When wiring between component corners, it is necessary to use solder pads with asymmetric lengths, which often work twice as much as half the power;

(3) The size of each component pad hole should be edited and determined according to the thickness of the component pins, with the principle that the hole size should be 0.2-0.4 millimeters larger than the pin diameter.

7. Various types of masks

These films are not only essential in the PcB manufacturing process, but also a necessary condition for component soldering. According to the position and function of the "film", it can be divided into two types: TOp or Bottom solder mask on the component surface (or welding surface) and TOp or Bottom Paste Mask on the component surface (or welding surface). As the name suggests, the solder mask is a layer of film applied to the solder pad to improve its solderability, which refers to the light colored circular spots slightly larger than the solder pad on the green board. The situation of solder mask is exactly the opposite. In order to make the made board suitable for wave soldering and other welding forms, it is required that the copper foil on the non solder pads of the board cannot be tinned. Therefore, a layer of coating should be applied to all parts outside the solder pads to prevent these parts from being tinned. It can be seen that these two types of membranes have a complementary relationship. From this discussion, it is not difficult to determine the menu

Settings for projects such as' Solder Mask En1argetion 'have been implemented.

 

8. Flying line

A network connection similar to a rubber band used for observation during automatic routing. After calling in the components through the network table and making a preliminary layout, the "Show" command can be used to see the intersection status of the network connections under this layout. The position of the components can be continuously adjusted to minimize this intersection and obtain the maximum routing rate of automatic routing. This step is very important and can be said to be a sharpening of the knife without cutting wood, spending more time and value! In addition, after the automatic routing is completed, which networks have not yet been routed can also be searched through this function. After finding the unconnected networks, manual compensation can be used. If it cannot be compensated, the second layer meaning of "flying lines" will be used, which is to connect these networks with wires on future printed boards. It should be noted that if the circuit board is produced in large quantities on an automated production line, this type of flying wire can be designed as a resistance element with a zero ohm resistance and uniform pad spacing

  

 

Hardware welding technology

★ Key points

Welding is an important step in repairing electronic products. After the fault detection of electronic products, the next step is welding.

There are several commonly used heating methods for soldering electronic products, including soldering iron, hot air, solder paste, infrared, laser, etc. Many large soldering equipment uses one or a combination of these heating methods.

Common welding tools include: soldering iron, hot air soldering station, tin furnace, BGA welding machine

Welding accessories: solder wire, rosin, soldering gun, solder paste, braided wire, etc.

Electric soldering iron is mainly used to solder discrete components of analog circuits, such as resistors, capacitors, inductors, diodes, transistors, field-effect transistors, etc. It can also be used to solder smaller QFP packaged integrated blocks. Of course, we can also use it to solder CPU broken pins and repair PCB boards. If the gold fingers of graphics cards or memory cards are damaged, electric soldering iron can also be used for repair. The heating core of an electric soldering iron is actually a resistance wire wound many times, and the power varies depending on the length of the resistance or the material used. Ordinary soldering irons for repairing electronic products generally use 20W-50W. Some high-end soldering irons are made into constant temperature soldering irons, and the temperature can be adjusted. There is an automatic temperature control circuit inside to maintain a constant temperature. This type of soldering iron has better performance, but the price is generally more expensive, more than ten or even dozens of times that of ordinary soldering irons. The melting point of pure tin is 230 degrees, but the solder we use for maintenance often contains a certain proportion of lead, resulting in its melting point being lower than 230 degrees, with the lowest generally being 180 degrees.

The newly purchased soldering iron needs to be tinned first. Soldering refers to sticking solder to the tip of the soldering iron so that it can be used normally. If the soldering iron is used for too long, the surface may oxidize due to high temperature. Oxidized soldering iron does not stick to tin, and it also needs to be tinned before it can be used normally.

Welding:

When dismantling or soldering resistors, capacitors, inductors, diodes, transistors, and field-effect transistors, some solder can be applied to the pins of the components to better transfer heat. When all the pins of the components are melted, they can be removed or soldered on. When soldering, pay attention to the high temperature. After melting, quickly lift the soldering iron tip to make the solder joint smooth. However, if the temperature is too high, it is easy to damage the solder pad or component.

Supplement PCB wiring

The situation of PCB board breakage occurs from time to time. The wires of displays, switch power supplies, etc. are relatively thick, and the broken wires are easy to repair. As for the wires of motherboards, graphics cards, and laptops, they are very thin and have small wire spacing, making it more difficult to repair them. To fix these broken wires, first prepare a very narrow flat blade scraper. The scraper can be manually ground on a grinding stone with a small screwdriver, so that the width of the scraper blade is about the same as the width of the PCB board wiring. When repairing wires, first use a scraper to scrape off the insulation paint on the surface of the PCB board where the wire is broken. Be careful not to use too much force to avoid scratching off the wire. In addition, be careful not to scrape off the insulation paint on the adjacent PCB wiring surface to prevent solder from sticking to the adjacent wire. After surface treatment, evenly apply a layer of solder paste on top, then use a soldering iron to heat and apply solder on the wire where the paint is scraped off. Then find the scrapped mouse, extract the fine copper wire inside, apply solder paste to a single copper wire, and then use a soldering iron to carefully solder the fine copper wire to both ends of the wire.

After welding is completed, a multimeter should be used to check the reliability of the welding. First, the two ends of the wire should be measured to confirm whether the wire has been connected, and then the repaired wire and the adjacent wire should be checked for adhesion and short circuit.

Repair of Plastic Soft Wire

The connection between the optical drive laser head cable and the printer print head often breaks, and the soldering method is similar to PCB board wiring. It should be noted that ordinary plastics can withstand very low temperatures. When soldering with a soldering iron, the temperature should be controlled and the speed should be as fast as possible to avoid plastic being burned out. In addition, to prevent thermal deformation, small clips can be used to clamp and position the wires.

 

Welding of CPU broken pins:

The situation of CPU pin breakage is very common. The roots of the pins of the 370 structure Celeron first generation CPU and P4 CPU are relatively sturdy, and the broken pins are usually broken from the middle, which is relatively easy to solder. As long as solder paste is applied to the corresponding place between the pin and the solder pad, and then heated with a soldering iron, it can be soldered on. For special positions that are inconvenient to use a soldering iron, a hot air soldering station can be used for heating.

When the pins of the second-generation Saiyang CPU are subjected to excessive external force, they often pull out completely, and the solder pads underneath are very small after pulling out. The success rate of direct soldering is low, and after soldering, the pins are not easy to fix and are easily knocked off. There are generally several ways to deal with this situation. The first way is to solder one end of the thin copper wire peeled from the mouse to the CPU's solder pad, and then use 502 glue to stick the wire to the CPU, and solder the other end to the corresponding solder pad on the motherboard CPU socket. In terms of electrical connection, it is no different from plugging into the motherboard. The disadvantage of Weiyi is that it is inconvenient to remove the CPU. The second method is to place a solder ball (which can be made by BGA soldering or by oneself) on the solder pad at the CPU pin breakage point, and then make a slightly longer pin by oneself (insert it into the CPU socket corresponding to the broken pin, fix a small piece of cured conductive adhesive (conductive adhesive has certain elasticity) on top, and then insert the CPU into the CPU socket, press and lock it tightly. This way, the processed CPU may work normally.

Welding of golden fingers such as graphics cards and memory modules:

If the graphics card or memory card is repeatedly unplugged or plugged in from the motherboard, it may cause the gold fingers to fall off, and the power or ground pins are often burned out due to excessive current. In order to make them function properly, the gold fingers need to be repaired. The repair of the gold fingers is relatively simple. You can scrape off the same gold fingers from other scrapped cards with a wallpaper knife, clean the surface, and carefully align them with 502 glue and stick them to the damaged card. After the glue solidifies, use a wallpaper knife to scrape off the oxide on the upper end of the newly attached gold fingers, apply solder paste, and then connect it to the broken wire with a fine copper wire.

Welding of integrated blocks:

In the absence of a hot air soldering station, it is also possible to consider using a soldering iron in conjunction with soldering to remove or solder the integrated block. The method is to use a soldering iron to fill all the pins of the chip with solder, and then use the soldering iron to cycle and heat the solder until all the pins are melted at the same time, and then the chip can be removed. To remove the chip from the circuit board, you can consider threading a thin copper wire under the pins of the chip and then lifting it up by hand from above.

Hot air welding station

The hot air soldering station achieves the soldering function by heating the solder with hot air. Inside the black box is an air pump with good performance and low noise. The function of the air pump is to continuously blow out air, and the airflow flows along the rubber tube to the front handle. Inside the handle is the heating core of the soldering station, which will generate heat when powered on. When the airflow inside comes out through the air nozzle, it will bring out the heat.

Each soldering station is equipped with multiple air nozzles, which are used in conjunction with different chips. In fact, most technicians can complete most soldering work with only one or two of these air nozzles, which is why circular holes are the most commonly used. According to our usage, the hot air welding station generally uses the 850 model, which has a maximum power consumption of 450W. There are two knobs on the front, one of which is responsible for adjusting the wind speed and the other for adjusting the temperature. Before use, the pump screws at the bottom of the body must be removed, otherwise it may cause serious problems. After use, remember to cool down the body. After turning off the power, the heating tube will automatically spray cool air briefly. During this cooling period, please do not unplug the power plug. Otherwise, it will affect the service life of the heating element. Note that the 850 air nozzle and the hot air it sprays have a high temperature during operation, which can burn people. Do not touch it. When replacing the air nozzle, wait for its temperature to drop before operating.

 

The following describes the replacement of QFP chips

Firstly, turn on the power, adjust the airflow and temperature control knob to maintain the temperature between 250-350 degrees. Place the puller under the integrated circuit block and align the nozzle with the pins of the chip to be melted for heating. When all the pins are melted, lift the puller and remove the chip. After removing the chip, you can apply an appropriate amount of solder paste on the solder pads of the circuit board, heat them with a nozzle to make them as flat as possible, and then apply an appropriate amount of solder paste on the solder pads. Align and fix the chip to be replaced on the circuit board, and then use a nozzle to evenly blow hot air towards the pins. After all the pins are melted, the soldering is completed. Finally, it is important to check whether the welded components are free from short circuits and virtual soldering.

BGA chip soldering:

The use of BAG chip mounting machines varies depending on the machine, and the accompanying manual provides detailed descriptions.

Replacement of slots (seats):

The size of the slot (socket) is relatively large, and wave soldering is generally used for welding on the production line. The wave soldering machine can melt the solder into solder paste and make the solder paste form waves. The peak of the waves contacts the lower surface of the PCB board, so that the slot (socket) and the solder pad are soldered together. For small-scale production or maintenance, a tin furnace is often used to replace the slot (socket). The principle of the tin furnace is similar to wave soldering, which uses solder paste to remove or solder the slot, as long as the soldering surface matches the slot (socket).

Disassembly and soldering techniques for surface mount components

The disassembly and soldering of surface mount components should use a 200-280 ℃ temperature regulating pointed soldering iron. The substrates of surface mount resistors and capacitors are mostly made of ceramic materials, which are prone to breakage due to collisions. Therefore, when disassembling and welding, it is necessary to master techniques such as temperature control, preheating, and light touch. Temperature control refers to controlling the welding temperature at around 200-250 ℃. Preheating refers to placing the components to be welded in an environment of around 100 ℃ for 1-2 minutes to prevent sudden thermal expansion and damage to the components. Lightly touching refers to the process where the soldering iron tip should first heat the solder joints or tape guides of the printed circuit board, and try not to touch the components. In addition, it is necessary to control the welding time at around 3 seconds each time, and allow the circuit board to cool naturally at room temperature after welding is completed. The above methods and techniques are also applicable to the soldering of surface mount crystal diodes and transistors.

SMT integrated circuits have a large number of pins, narrow spacing, and low hardness. If the soldering temperature is not appropriate, it can easily cause pin soldering short circuits, virtual soldering, or copper foil detachment from the printed circuit board. When disassembling a surface mounted integrated circuit, the temperature of the temperature regulating soldering iron can be adjusted to around 260 ℃. After using the soldering iron tip and a solder suction device to remove all the solder from the pins of the integrated circuit, gently insert pointed tweezers into the bottom of the integrated circuit, heat the pins with the soldering iron, and gently lift them one by one with tweezers to gradually detach them from the printed circuit board. When lifting an integrated circuit with tweezers, be sure to synchronize with the area heated by the soldering iron to prevent damage to the circuit board caused by hasty operation.

Before replacing with a new integrated circuit, all the solder left by the original integrated circuit should be removed to ensure the flatness and cleanliness of the solder pads. Then polish and clean the pins of the integrated circuit to be soldered with fine sandpaper, evenly tin them, and align the pins of the integrated circuit to the corresponding solder joints on the printed board. During soldering, gently press the surface of the integrated circuit with your hand to prevent it from moving. Use the other hand to operate the soldering iron dipped in an appropriate amount of solder to fix the pins at the four corners of the integrated circuit to the circuit board. After checking and confirming the model and direction of the integrated circuit again, solder it formally. Adjust the temperature of the soldering iron to around 250 ℃, hold the soldering iron with one hand to heat the pins of the integrated circuit, and use the other hand to send the solder wire to the heated pins for soldering until all pins are heated and soldered. Finally, carefully check and eliminate pin short circuits and virtual soldering. After the solder joints cool naturally, use a brush dipped in anhydrous alcohol to clean the circuit again. Board and solder joints to prevent residual welding slag.

Before troubleshooting the module circuit board, it is advisable to use a brush dipped in anhydrous alcohol to clean the printed circuit board, remove dust, solder slag, and other debris from the board, and observe whether there is virtual soldering or solder slag short circuit on the original circuit board, as well as detect the fault point early to save maintenance time.

  

 

BGA solder ball reset process

★ Understand

1. Introduction

BGA, as a high-capacity packaged SMD, has promoted the development of SMT. Manufacturers and manufacturers have realized that BGA has strong vitality and competitiveness in high-capacity pin packaging. However, BGA individual devices are expensive, and there are often multiple tests for pre developed products. It is often necessary to remove BGA from the substrate and hope to reuse the device. Due to the damage to the solder balls of BGA after removal, they cannot be directly soldered onto the substrate and must be repositioned. The technical challenge of how to regenerate the solder balls lies in front of our process technicians. At Indium, BGA specific solder balls can be purchased, but the process of repairing each solder ball individually is clearly not feasible. This article introduces a SolderQuick preform technology for ball regeneration of BGA.

2. Equipment, tools, and materials

Pre formed bad fixture soldering flux deionized water cleaning tray cleaning brush 6-inch flat tweezers acid resistant brush reflow soldering oven and hot air system microscope finger cots (some tools can be selected according to specific situations)

3. Process flow and precautions

3.1 Preparation

Confirm that the BGA fixture is clean. Heat the reflow soldering furnace to the temperature required by the temperature curve.

3.2 Process steps and precautions

3.2.1 Place the pre formed parts into the fixture

Place the pre formed part into the fixture, with the side labeled SolderQuik facing downwards towards the fixture. Ensure that the preform is loose fit with the fixture. If the preform is damaged and needs to be bent before being installed into the fixture, it cannot proceed to the subsequent process. The main reason why the preform cannot be placed in the fixture is due to dirt on the fixture or improper adjustment of the flexible fixture.

3.2.2 Apply an appropriate amount of soldering flux on the repaired BGA

Apply a small amount of soldering flux to the BGA soldering surface that needs to be repaired using an injection syringe containing soldering flux. Attention: Confirm that the BGA soldering surface is clean before applying soldering flux.

3.2.3 Apply the flux evenly and use an acid resistant brush to evenly brush the flux on the entire soldering surface of the BGA package, ensuring that each solder pad is covered with a thin layer of flux. Ensure that each solder pad has solder flux. The soldering effect of thin flux is better than that of thick flux.

3.2.4 Place the BGA to be repaired into the fixture, with the side coated with flux facing the preform.

3.2.5 Level the BAG and gently press the BGA to position the preform and BGA in the fixture, ensuring that the BGA is placed flat on top of the preform.

3.2.6 Reflow soldering

Place the fixture into a hot air convection furnace or hot air recirculation station and start the reflux heating process. All reflow station curves used must be set as curves specifically designed for BGA solder ball regeneration processes that have been developed.

3.2.7 Cooling

Use tweezers to remove the fixture from the furnace or reflow station and place it on the heat transfer plate, cool for 2 minutes.

3.2.8 Removal

After the BGA cools down, remove it from the fixture and place its solder ball face up in the cleaning tray.

3.2.9 Soaking

Soak the BGA in deionized water for 30 seconds until the paper carrier is fully immersed before proceeding to the next step.

3.2.10 Peel off the solder ball carrier

Use specialized tweezers to remove the solder balls from the BGA. The best way to peel off is to start from one corner. The peeled paper should be intact. If the paper is torn during the peeling process, stop immediately and add some deionized water. Wait for 15 to 30 seconds before continuing.

3.2.11 Remove paper scraps from BGA. Occasionally, a small amount of paper scraps may remain after peeling off the carrier. Use tweezers to remove the paper scraps. When using tweezers to pick up paper scraps, the tweezers should be gently moved between the solder balls. Caution: The head of the tweezers is very sharp, and if you are not careful, you may scratch the fragile solder mask.

3.2.12 Cleaning

After removing the paper carrier, immediately wash the BGA in deionized water. Rinse with plenty of deionized water and brush the BGA diligently.

Caution: When brushing with a brush, support the BGA to avoid mechanical stress.

Attention: To achieve the best cleaning effect, brush in one direction, then turn it 90 degrees, brush in another direction, then turn it 90 degrees, brush in the same direction until it turns 360 degrees.

3.2.13 Rinse

Rinse BGA in deionized water to remove a small amount of residual flux and paper scraps from the previous cleaning steps. Then air dry it, do not use dry tissues to wipe it dry.

3.2.14 Inspection of packaging

Inspect the package under a microscope for contamination, absence of solder balls, and residual flux. If cleaning is required, repeat steps 3.2.11-3.2.13.

Attention: As the flux used in this process is not a cleaning free flux, careful cleaning to prevent corrosion and long-term reliability failure is necessary.

The best way to determine if the packaging is clean is to test ion contamination using ionization diagrams or efficiency equipment. The test results of all processes must meet the standard of contamination below 0.75mg NaaCI/cm2. In addition, the cleaning steps of 3.2.9-3.2.13 can be replaced by sink cleaning or spray cleaning processes.

4. Conclusion

Due to the high cost of devices on BGA, BGA rework has become essential, with key solder ball regeneration being a technical challenge. This process is practical and reliable, and only requires the purchase of pre formed parts and fixtures for BGA soldering regeneration. This process solves the key technical problems in BGA repair

 

Analysis of Common Problems in the Use of Solder Paste

★ Key points

Reflow soldering of solder paste is the main board level interconnect method used in SMT assembly processes. This soldering method combines the required soldering characteristics very well, including ease of processing, wide compatibility with various SMT designs, high soldering reliability, and low cost; However, when reflow soldering is used as the most important SMT component level and board level interconnect method, it is also challenged by the need to further improve soldering performance. In fact, whether reflow soldering technology can withstand this challenge will determine whether solder paste can continue to be the primary SMT soldering material, especially with the continuous progress of ultra-fine spacing technology. Below, we will explore several main issues that affect the improvement of reflow soldering performance. In order to stimulate the industrial community to develop new methods to solve this problem, we will briefly introduce each issue separately.

Fixing of bottom components

Double sided reflow soldering has been used for many years. Here, the first side is printed and wired, components are installed, and soft melted. Then, the other side of the circuit board is processed by flipping it over. In order to save more money, some processes omit the soft melting of the first side and instead soft melt both the top and bottom sides. A typical example is when only small components, such as chip capacitors and chip resistors, are installed on the bottom side of the circuit board. As the design of printed circuit boards (PCBs) becomes increasingly complex, the components installed on the bottom side also become larger. As a result, component detachment during soft melting becomes an important problem. Obviously, the phenomenon of component detachment is due to insufficient vertical fixing force of the melted solder during soft melting, which can be attributed to the increase in weight of the component, poor solderability of the component, insufficient wetting of the solder flux, or insufficient amount of solder. Among them, the first factor is the most fundamental reason. If there is still component detachment after improving the following three factors, SMT adhesive must be used. Obviously, the use of adhesive will deteriorate the self alignment effect of the components during soft melting.

Not fully welded

Incomplete soldering forms a solder bridge between adjacent leads. Usually, all factors that can cause solder paste to collapse will lead to incomplete soldering, including: 1. too fast heating rate; 2. Poor thixotropy of solder paste or slow recovery of solder paste viscosity after shearing; 3. Metal load or solid content is too low; 4. The particle size distribution of the powder is too wide; 5； The surface tension of the solder flux is too low. However, collapse does not necessarily cause incomplete welding. During soft melting, the melted incomplete welding material may break under the push of surface tension, and the phenomenon of solder loss will make the problem of incomplete welding even more serious. In this case, excessive solder that accumulates in a certain area due to solder loss will cause the molten solder to become too much and difficult to break.

In addition to the factors that cause solder paste to collapse, the following factors are also common causes of incomplete soldering: 1. Compared to the space between solder joints, there is too much solder paste deposition; 2. Heating temperature is too high; 3. Solder paste heats up faster than circuit boards; 4. The wetting speed of the solder flux is too fast; 5. The vapor pressure of the solder is too low; 6； The solvent composition of the solder is too high; 7. The softening point of the flux resin is too low.

Intermittent wetting

Intermittent wetting of solder film refers to the presence of water on a smooth surface (1.4.5.). This is because solder can adhere to most solid metal surfaces and hide some unwetted points under the melted solder coating. Therefore, intermittent wetting occurs when initially covering the surface with melted solder. The metastable molten solder coating will shrink under the minimum surface energy driving force, and soon aggregate into separated small balls and ridge like protrusions. Intermittent wetting can also be caused by the gas released when the component comes into contact with melted solder. The water released due to the thermal decomposition of organic matter or the hydration of inorganic matter will produce gas. Water vapor is the most common component of these gases. At the welding temperature, water vapor has a strong oxidizing effect and can oxidize the surface of the molten solder film or some interfaces below the surface (a typical example is the metal oxide surface at the interface of the molten solder). A common situation is that higher welding temperatures and longer residence times can lead to more severe intermittent wetting phenomena, especially in the base metal, where an increase in reaction rate can result in more intense gas release. Meanwhile, a longer residence time will also prolong the time for gas release. Both of the above aspects will increase the amount of gas released, and the method to eliminate intermittent wetting phenomenon is: 1. Reduce the welding temperature; 2. Shorten the residence time of soft melting; 3. Adopt a flowing inert atmosphere; 4. Reduce the level of pollution.

Low residue

For the soft melt process that does not require cleaning, low residue is often required to achieve decorative or functional effects. Examples of functional requirements include "exploring and testing the weld layer by testing the flux residue in the circuit, and making electrical contact between the insertion joint and the weld layer or between the insertion joint and the through hole near the soft melt welding point". Excessive flux residue often leads to excessive residue coverage on the metal surface where electrical contact is to be made, which can hinder the establishment of electrical connections. With the increasing density of circuits, this issue has attracted more attention.

Obviously, low residue solder paste that does not require cleaning is an ideal solution to meet this requirement. However, the necessary conditions for melting related to this have made this issue even more complex. In order to predict the welding performance of low residue solder paste in different levels of inert soft melt atmosphere, a semi empirical model is proposed. This model predicts that as the oxygen content decreases, the welding performance will rapidly improve and then gradually stabilize. Experimental results show that with the decrease of oxygen concentration, the welding strength and wetting ability of solder paste will increase. In addition, the welding strength also increases with the increase of solid content in the flux. The model proposed by the experimental data is comparable and strongly proves its effectiveness in predicting the welding performance of solder paste and materials. Therefore, it can be concluded that in order to successfully use low residue solder without cleaning in the welding process, an inert soft melt atmosphere should be used.

Gap

Gap refers to the absence of solder joints between component leads and circuit board solder joints. Generally speaking, this can be attributed to the following four reasons: 1. Insufficient solder deposition; 2. Poor coplanarity of leads; 3. Insufficient wetting; 4. Solder loss - This is caused by the collapse of solder paste on pre tinned printed circuit boards, the core suction effect of leads (2.3.4), or through holes near solder joints. The problem of lead coplanarity is a particularly concerning issue for new lightweight 12 mil (μ m) pitch quad flat packs (QFP). To solve this problem, a method of pre coating solder joints with solder before assembly (9) has been proposed. This method expands the size of local solder joints and forms a controllable local welding area along the bulging solder pre coverage area, thereby compensating for changes in lead coplanarity and preventing gaps. The core suction of leads The effect can be solved by slowing down the heating rate and allowing the bottom surface to receive more heat than the top surface. In addition, using solder with a slower wetting rate, A higher activation temperature or solder paste that can delay melting (such as solder paste mixed with tin powder and lead powder) can also minimize chip suction. Before using a tin lead coating to polish the circuit board, covering the connection path with a solder mask can also prevent chip suction caused by nearby through holes.

 

Solder into balls

The formation of solder balls is the most common and challenging problem, which refers to the solidification of solder into spherical particles of varying sizes not far from the main solder pool during the softening process; In most cases, these spherical particles are composed of solder powder in solder paste. The formation of solder balls raises concerns about issues such as circuit short circuits, leakage, and insufficient solder at solder joints. With the advancement of fine spacing technology and non cleaning soldering methods, there is an increasing demand for the use of SMT processes without solder ball formation.

The reasons for the formation of solder balls (1, 2, 4, 10) include: 1. Oil stains caused by improper circuit printing processes; 2. Excessive exposure of solder paste to an oxidizing environment; 3. Excessive exposure of solder paste to humid environments; 4. Inappropriate heating methods; 5. Heating speed is too fast; 6. The preheating section is too long; 7. Interaction between solder mask and solder paste; 8. Insufficient flux activity; 9. Excessive welding powder oxide or pollution; 10. Too many dust particles; 11. Improper volatile substances are mixed into the flux during specific melting treatments; 12. Solder collapse caused by improper solder paste formula; 13. Before using solder paste, it was opened and used without fully recovering to room temperature; 14. Excessive printing thickness leads to "collapse" and the formation of solder balls; 15. The metal content in solder paste is relatively low.

 

Solder bead

Solder bead formation is a special phenomenon in which solder balls are formed during the use of solder paste and SMT processes, Simply put, solder balls refer to very large solder balls with (or without) small solder balls (11) attached to them. They are formed around components with extremely low support legs, such as chip capacitors. Solder bead formation is caused by the exhaust of solder flux. During the preheating stage, this exhaust effect exceeds the cohesive force of the solder paste, and the exhaust promotes the formation of isolated clusters of solder paste under low gap components. During melting, the melted isolated solder paste emerges again from under the components and aggregates.

The reasons for welding beads include: 1. The thickness of the printed circuit is too high; 2. There is too much overlap between solder joints and components; 3. Too much solder paste was applied under the component; 4. The pressure of installing components is too high; 5. The temperature rises too quickly during preheating; 6. The preheating temperature is too high; 7. Moisture is released from the components and solder resist; 8. The activity of the solder is too high; 9. The powder used is too fine; 10. The metal load is too low; 11. Too much collapse of solder paste; 12. Too much solder powder oxide; 13. Insufficient solvent vapor pressure. The simplest method to eliminate solder bead formation may be to change the shape of the template pores, so that there is less solder paste sandwiched between the low mount components and solder joints.