Repair technology and operation methods of BGA components

Ball Grid Array (BGA) packaging technology has been used in cutting-edge armaments, missiles and aerospace technology as early as the 1980s.

With the development of semiconductor process technology, BGA packaged IC components have been widely used in mobile phones in recent years, which plays a decisive role in the miniaturization and multifunctionality of mobile phones. However, mobile phone manufacturers also take advantage of the difficulty of repairing BGA components and artificially add certain restrictions to restrict the mobile phone repair industry, which makes us encounter certain difficulties in the process of repairing BGA, and even have no idea where to start. The author has been engaged in electronic communication repair for ten years and has a little knowledge of SMD component welding and desoldering equipment. I would like to introduce the repair technology and operation skills of BGA components to my peers, hoping to "throw a brick to attract jade" and improve the technology to a higher level.

1. Issues that need to be paid attention to in BGA maintenance

Because of the inherent characteristics of BGA packaging, the following issues should be kept in mind:
① Prevent overheating damage during soldering and disassembly.
② Prevent static electricity accumulation damage.
③ Airflow and pressure of hot air soldering.
④ Prevent damage to the BGA pad on the PCB.
⑤ Positioning and direction of BGA on the PCB .
⑥ Performance of tin-planted steel sheet.
The assembly and soldering of BGA on PCB is originally carried out by automated equipment in electronic factories. Although it is difficult to encounter the above problems in amateur situations, with a careful, rigorous and scientific attitude, and with the help of advanced rework equipment and tools, it is easy to operate and the success rate is relatively high.

2. Basic equipment and tools used in BGA repair

The success or failure of BGA repair is largely determined by the tinning tools and "hot air gun". The most common problems encountered by the author and many colleagues are the difficulty of tinning and the "unpredictable" operating temperature and wind pressure of "850". Even if the "white light" 850 hot air gun is used, the temperature and airflow are difficult to control due to voltage fluctuations, and the BGA and motherboard are damaged unknowingly, so the success rate is not high. After several screenings from the repair equipment market, the author finally selected the following equipment and tools from the perspective of accuracy, reliability and scientificity (as shown below):
① SUNKKO 852B intelligent hot air desoldering machine.
② SUNKKO 202 BGA anti-static tinning repair station.
③ SUNKKO BGA special welding nozzle.
④ SUNKKO 3050A anti-static cleaner.
The vacuum suction pen and magnifying glass (microscope is better) are used as auxiliary.

3. BGA maintenance operation skills

⑴. Preparation before BGA desoldering.
Set the parameters of SUNKKO 852B as follows: temperature 280℃~310℃; desoldering time: 15 seconds; air flow parameters: ××× (1~9 levels can be preset through user code); (as shown below)

Finally, set the desoldering machine to the automatic mode, use the SUNKKO 202 BGA anti-static tinning repair station, use the multi-purpose tip to install the mobile phone PCB board and fix it on the repair station.
⑵. Desoldering.
Remember the direction and positioning of the chip before desoldering. If there is no positioning frame printed on the PCB, use a marker to mark it around the edges, inject a small amount of flux at the bottom of the BGA, select a BGA special welding nozzle of the appropriate size of the BGA to be desoldered and install it on the 852B.

Align the handle vertically to the BGA, but note that the nozzle must be about 4mm away from the component. Press the start button on the 852B handle, and the desoldering machine will automatically desolder with the preset parameters.

After the desoldering is completed, remove the BGA component with a suction pen after 2 seconds, so that the original solder balls can be evenly distributed on the pads of the PCB and BGA, which is convenient for subsequent BGA soldering. If there is excess tin overlap on the PCB pad, use an anti-static soldering station to evenly treat it. For serious overlap, you can apply flux on the PCB again, start 852B again to heat the PCB, and finally make the tin package neat and smooth. Use a tin-absorbing belt on the anti-static soldering station to completely absorb the tin on the BGA. Pay attention to anti-static and do not overheat, otherwise it will damage the pads and even the motherboard.
⑶. Cleaning of BGA and PCB.
Use high-purity board washing water to clean the PCB pads, use an ultrasonic cleaner (with an anti-static device) to load the board washing water, and clean the removed BGA.

⑷. Tinning of BGA chips.
Tinning of BGA chips must be done on a laser-punched steel sheet with a single-sided trumpet-shaped mesh. The thickness of the steel sheet is required to be 2mm, and the hole wall must be smooth and even.

The bottom of the speaker hole (the side that contacts the BGA) should be 10μm~15μm larger than the top (the small hole where the tin is scraped in). (The above two points can be observed through a ten-fold magnifying glass). In this way, the tin paste can be easily dropped onto the BGA without printing. It should be noted that most of the inferior steel sheets on the market are not processed by laser, but by chemical etching. In addition to the rough and irregular hole walls, the mesh is not horn-shaped or has double-sided horn holes. This type of steel sheet will encounter great difficulties when tinning. Use the tinning function of the above-mentioned BGA repair station → template and steel sheet, first find the corresponding concave position in the positioning template,
glue the BGA component into the concave with double-sided adhesive, put the steel sheet with precision positioning square and round holes on the positioning template, and then use its attached magnetic pressure block to press the steel sheet onto the template. The unique triple precision positioning device (BGA→template→steel sheet) ensures that even with eyes closed, the mesh of the steel sheet can be easily and accurately aligned with the small pad of the BGA component (but remember that the side with words engraved on it should face upwards). Use a small scraper to scrape a small amount of thick solder paste into the mesh of the steel sheet.

After the hole is filled, slowly lift the steel sheet from one end to remove the BGA chip.

If a small tin pile is missed, use the desoldering machine to heat it again as mentioned above, so that the tin pile on the BGA becomes a uniform array of tin balls. If there is no tin ball on some pads, you can press the steel sheet to make local tin filling. The author does not agree with the method of heating the steel sheet together, because this will not only affect the solder ball planting, but also cause the precision steel sheet to be deformed and damaged by heat.

⑸. Soldering of BGA chips.
Apply a small amount of thick flux (high purity is required, active rosin can be added to analytical pure alcohol to dissolve it) on the BGA solder ball and PCB pad, find the original mark and place the BGA. While soldering, the BGA can be bonded and positioned to prevent it from being blown away by hot air, but be careful not to put too much flux, otherwise the chip will shift due to excessive bubbles generated by rosin during heating. The PCB board is also placed in an anti-static maintenance table and fixed with a multi-purpose top and must be placed horizontally. The parameters of the intelligent desoldering machine are preset to a temperature of 260℃~280℃, a welding time of 20 seconds, and the airflow parameters remain unchanged. When the BGA nozzle is aimed at the chip and 4mm away, the automatic welding button is triggered. As the BGA solder ball melts, it forms a better tin alloy weld with the PCB pad, and the surface tension of the solder ball makes the chip automatically centered even if it is originally deviated from the motherboard, and the job is done. Note that no pressure should be applied to the BGA during soldering. Even if the wind pressure is too high, it will cause a short circuit between the solder balls under the BGA.

Single-sided speaker hole tin printing

4. Analysis of BGA welding process

Everyone knows that BGA components are "delicate" and PCB motherboards are "tender". They are extremely precise multi-layer three-dimensional cross-wiring. Most of the cross-layer wiring is only a fraction of a hair's width. Therefore, it is often the key to determine the success or failure of BGA during the welding process. The operation and adjustment of the "desoldering machine" are very important. The first is the temperature, and the second is the airflow and wind pressure. They must be clear and cannot be casual. Otherwise, all previous efforts will be wasted, and even irreparable damage will be caused. Therefore, you must be extra careful when using the traditional 850-type "hot air gun". It is best to use a thermometer to measure the hot air temperature first, and never use the BGA circuit board to test it. Why do many maintenance personnel find it too difficult to weld BGA? The main reason is that they cannot master the hot air parameters of the "hot air gun". Relying on subjective judgment or "experience" to weld high-requirement, high-precision BGA components is very imprecise from a practical and scientific point of view, and the chance of success is naturally low.
Although the over-temperature soldering operation is very fast, it brings two problems: ①
The alloy components formed by the BGA solder ball and the PCB pad become brittle and hard, and due to high-temperature oxidation, a loose grain structure is generated. When the PCB is installed, the oxidized solder ball is easily broken by force, resulting in poor solder joints.
② Over-temperature will cause the "tender" multi-layer wiring motherboard to be partially overheated and deformed, and in severe cases, it will bubble and delaminate and become scrapped. At the same time, many repairers currently use "hot air guns" to blow solder BGAs because they do not have special BGA nozzles. They simply remove the original nozzles of the "hot air guns" to operate. Since the hot air range is very large at this time, the SMD components next to the BGA are often heated together, resulting in the BGA being soldered, but the heat-resistant components around it (such as tantalum electrolytic capacitors, ceramic components, etc.) are burned. Using a special BGA nozzle can safely avoid this problem. Excessive air flow and pressure applied to the BGA chip is equivalent to external force pressing on the chip. Since the BGA chip is light and thin, it is possible to cause the solder ball to overlap inside, and all previous efforts will be wasted. Therefore, in the process of BGA welding, it is particularly important to scientifically set the appropriate temperature, air flow, time, and welding area parameters.

5. Inspection after BGA soldering and cleaning of PCB motherboard

Non-professionals do not have X-ray detection equipment for checking BGA welding quality, so we can only use magnifying glass lamp to check BGA components that have been soldered to PCB, mainly to check whether the chip is centered, whether the angle is corresponding, whether it is parallel to the PCB, whether there is solder overflow from the periphery, or even short circuit, etc. Otherwise, we have to re-solder and never power on the machine hastily to avoid expanding the fault surface. Only when the inspection is correct can we power on to check the electrical performance and function. In addition, after powering on to check that the performance and function are normal, the BGA components and PCB should be ultrasonically cleaned to remove excess flux and possible tin scraps.

BGA components are installed on the mobile phone motherboard. In addition to soldering the PCB with solder balls, strong glue is injected into the gap between the BGA and the PCB for reinforcement. In our repair process, how to remove the glue to facilitate tinning and soldering is one of the skills; in the process of BGA or PCB desoldering, due to the difficulty in controlling the temperature of the "hot air gun", the pads are often damaged or even fall off. What should we do when encountering this problem? This is the second skill; in the future, with the development of mobile phone design technology, its internal circuits will be integrated into a single-chip BGA on a large scale, which will increase the size of the BGA a lot. In fact, some models are already like this. When repairing such large BGAs, we should master those more scientific methods instead of letting readers guess what "stunts" are. This is the third technology. The following will introduce them one by one.

1. Glue treatment
According to the author's knowledge, the glue used between the mobile phone motherboard and BGA is basically of three types: ① ether glue. ② epoxy resin glue. ③ polyester glue. These glues may be two-component cured or UV cured during production and construction. Therefore, it is indeed difficult to perform sol treatment on them. The effective method we have adopted now is: select the corresponding sol water and select non-corrosive and low-volatile ones. For desoldering BGA and removing glue at the same time, use an intelligent desoldering device to control the temperature to separate BGA from the board first, and soften the glue at a suitable temperature. However, it should be noted that BGA must not be forcibly removed before the glue softens, otherwise it will be damaged. The actual effect of BGA sol water on the market is not ideal, mainly because the sol time is too long (some need 2-3 hours). The author uses the "boiling soup" method to greatly accelerate the dissolution of glue: find a small metal box, such as the iron box of "cooling oil", clean it and use it as a "pot". When "boiling soup", put the BGA component with glue in, then pour a small amount of sol water, and close the lid. The "pot" is placed on the soldering iron part of the constant temperature soldering station, and the temperature is adjusted to 200℃~250℃. The soldering iron will heat the melted glue and accelerate the separation of the glue on the BGA. But please note that if the volatile sol water is heated, it will cause the metal box cover to be washed off, so be careful. For the remaining glue on the motherboard, use a constant temperature desoldering device at a fixed temperature of 250℃~280℃ to heat it first, then apply melted glue, and repeat it many times until it is removed.

2. Repair of damaged pads on BGA or PCB
Both BGA and PCB pads are made of copper-silver alloy deposited by electroforming. They have sufficient mechanical strength at room temperature, but are easy to fall off under the influence of temperature, especially overheating. My method of repairing them is very simple and reliable:
① Observe through a magnifying glass or microscope, use a sharp blade to carefully scrape a small concave round pit left by the "root" of the removed pad, the size is slightly larger than the original pad, about 0.1mm deep, and scrape out the metallic luster of the "root";

② Use a small amount of high temperature resistant conductive glue to apply (to repair a pad identical to the original pad, you can use the reverse side of the BGA tin-planted steel sheet to press it on and then use hot air at about 150°C for about 20 minutes to make a repair pad that can withstand 300°C high temperatures and conduct electricity. Some articles introduce the use of thin copper wires to wrap around pads and then use tin to solder "roots" the size of a hair. Even if they can be soldered, how strong is the mechanical adsorption? And when the BGA is subsequently soldered to the motherboard, such a "pad" is very easy to fall off when heated, resulting in all previous efforts being wasted. (Regarding high temperature resistant conductive glue, if readers need it, please contact the author)

3. Rework precautions and methods for large monolithic BGA chips
Large monolithic BGA chips usually refer to ICs that are larger than the head of the "desoldering machine". I often encounter them when repairing laptops, and now mobile phones are also beginning to use them. Perhaps, some people think that the rework of small BGAs and large BGAs can be done in the same way, which is wrong. The reason is that small BGAs have a small area and are relatively "point" type, and their stress on the PCB can be ignored; while large BGAs have a large area and are relatively "surface" type, and their stress on the PCB needs to be considered. What will happen if the stress increases? Think about it, when the BGA steel sheet is heated, the larger the steel sheet is, the greater the deformation is. This is the effect of stress. If the mobile phone motherboard you are repairing has stress deformation due to large-area heating, then you can imagine what the consequences are. Therefore, when encountering these machines, you should pay attention to three points: 1) Pay attention to the temperature setting; 2) Be sure to use a BGA-specific nozzle, and don't blow it randomly over a large area; 3) Use the method of heating up and down at the same time to eliminate and reduce thermal stress. The specific method is as follows: Use a BGA repair platform to fix the motherboard horizontally, place a SUNKKO 853 auxiliary preheating table under the BGA to be removed, set the temperature to 200℃~220℃, and keep the temperature constant for 1 minute. Use SUNKKO 852 above the BGA and install a BGA nozzle. Select the setting parameters (see the previous introduction), and you can safely and quickly remove and solder large BGA components.

4. Postscript (note)
Repairing BGA components is a highly technical and skillful operation. It must not be done carelessly. We must remember to be careful, rigorous and scientific. We should use advanced repair equipment as much as possible. We should learn and adopt advanced processes. Don't be confused by some "homemade methods". Otherwise, you will "lose both the wife and the army", magnify the original minor fault, and even scrap the motherboard. The author believes that with the continuous research and innovation of the industry's experts, the repair technology of mobile phone BGA will definitely be more advanced, easier and more scientific.