preface

With the development of electronic products in the direction of miniaturization, portability, networking and high performance, higher requirements are put forward for circuit assembly technology and the number of I/O leads. The size of the chip is getting smaller and smaller, and the number of pins of the chip is increasing, which brings difficulties to production and repair.

Quadrilateral flat package QFP was widely used in SMT, and the limit size of package spacing remained at 0.3 mm. Leads with such spacing were easy to bend, deform or break. The requirements for SMT assembly process, equipment accuracy and welding materials were high, and the defect rate of QFP with narrow assembly and fine spacing was up to 6000 ppm, which restricted the wide range of applications. For BGA devices packaged with ball grid array, because the pins of the chip are distributed on the bottom of the package, it is easy to use SMT to weld and interconnect with the wiring pins on the PCB by changing the pins from the original four sides of the package shell substrate into lead/tin convex pins in a plane array layout, which can accommodate more I/O numbers, and replacing the 0.4 and 0.3 mm spacing of the QFP with larger pin spacing (such as 1.5 and 1.27 mm), Not only can the chip maintain more packaging capacity under the same packaging size as the QFP, but also the I/O pin spacing is larger, which greatly improves the yield of SMT assembly, with the defect rate of only 0.35 ppm, which is convenient for production and repair, so BGA has been widely used in the field of electronic product production.

In order to improve the quality and reliability of BGA welded joints, the defect performance and reliability of BGA welded joints were studied.

2、

The BGA welding quality and the inspection of the BGA welding spot are under the wafer. After welding, it is difficult to judge the welding quality with the naked eye. In the absence of detection equipment, you can first check whether the collapse of the outer ring of the chip is consistent, and then aim the chip at the light. If each row and each column can be transparent, then it is preliminarily judged that there is no welding. However, it is impossible to determine whether there are other defects in the inner solder joint or whether there are holes on the surface of the solder joint by this method. In order to judge the quality of solder joints more clearly, X-ray detection instruments must be used.

The commonly used X-ray detection instruments include two-dimensional X-ray direct radiography and X-ray circuit board detector. The traditional two-dimensional X-ray direct radiography equipment is relatively cheap. The disadvantage is that all solder joints on both sides of the PCB board are projected on the same photo at the same time. When there are components on both sides of the same position, the shadows formed by these solder joints will overlap, and it is not clear which side of the component is. If there are defects, it is also not clear which layer of the problem is, which cannot meet the requirements of accurately determining the welding defects.

The X-ray circuit board detector is an X-ray tomography equipment specially used to check the solder joints. It can not only check BGA, but also check all the solder joints on the PCB board. This equipment uses X-ray tomography, through which tin balls can be layered to produce tomography effect. The X-ray tomography can be compared according to the original CAD design data and the parameters set by the user, so that the conclusion of whether the welding is qualified or not can be drawn timely. Its disadvantage is that it is too expensive.

2．1

Acceptance standard of BGA solder joint No matter what equipment is used for inspection, there must be a standard to judge whether the quality of BGA solder joint is qualified. 12.2.12 of IPC-A-610C defines the acceptance criteria for qualified BGA solder joints as follows: the solder joints are smooth, round, with clear boundaries, without holes, the diameter, volume, gray level and contrast of all solder joints are the same, the positions are aligned, without deviation or torsion, and without solder balls. After welding, the preferred scheme to judge whether the welding spot is qualified or not is to meet the above requirements, but the standard can be slightly relaxed during actual inspection. If the position is aligned, the BGA solder joint is allowed to have an offset of no more than 25% from the pad, and the solder ball is not allowed to exist, but the solder ball cannot be greater than 25% of the distance between the two nearest solder balls.

2．2

Common defects of BGA welding Common defects of BGA welding: tin connection, open circuit, solder ball missing, hole, large solder ball and fuzzy edge of solder joint. Holes are not unique to BGA. The solder joints of surface-mounted and through-hole plug-in components can usually be seen visually without X-ray inspection. However, in BGA welding, because the solder joints are hidden under the package, only X-ray can be used to check whether these solder joints have holes. They even think that holes are good for reliability. IPC-7095 Committee believes that some holes that are very small and cannot be completely eliminated may be beneficial to reliability, but there should be a defined standard for how large the size is.

3、

The solder balls of the hole formation mechanism BGA include the component layer (the substrate near the BGA component), the pad layer (the substrate near the PCB) and the intermediate layer. According to different situations, voids can occur in any of the three layers. The BGA solder ball may have holes before welding, which will form after the reflow process is completed. This may be caused by the introduction of holes in the solder ball production or the solder paste material applied on the PCB surface.

In addition, PCB design is also the main reason for the formation of holes. For example, if the through hole is designed under the pad, during the welding process, the outside air enters the molten solder ball through the through hole. After the welding is completed and cooled, the solder ball will leave a hole. The voids in the pad layer may be due to the volatilization of the flux in the solder paste printed on the pad during reflow welding, and the gas escapes from the molten solder, forming voids after cooling.

Poor gold plating of the pad or contamination on the surface of the pad will cause voids. It is usually found that the most likely location of voids is in the component layer, that is, the part between the center of the solder ball and the BGA substrate. This may be due to the existence of air bubbles and volatile flux gas in the bonding pad of BGA on the PCB during reflow soldering. When the eutectic solder ball of BGA and the applied solder paste are integrated in the reflow soldering process, voids are formed. If the reflow temperature curve is not long enough in the reflow zone, the air bubbles and volatile flux gas can not escape in time, and the molten solder has entered the cooling zone and become solid, forming voids. Therefore, the setting of reflow temperature curve is an important reason for cavity formation.

4、

Suggestions for improving BGA welding reliability

1) The circuit board and chip shall be preheated to remove moisture. The BGA packaged on the tray shall be baked at 120 ℃ for 4~6 hours before welding.

2) Clean the solder pad and remove the flux and solder paste left on the PCB surface.

3) Fresh auxiliary materials must be used to apply solder paste and flux. The solder paste must be evenly mixed. The viscosity of solder paste and the amount of solder paste applied must be appropriate to ensure no continuous welding during the melting process of solder.

4) When mounting, each solder ball on the BGA chip must be aligned with each corresponding solder joint on the PCB.

5) During reflow soldering, the heating temperature and time of each zone should be correctly selected, and the heating speed should be paid attention to. Generally, before 100 ℃, the maximum heating rate shall not exceed 6 ℃/s, after 100 ℃, the maximum heating rate shall not exceed 3 ℃/s, and in the cooling zone, the maximum cooling rate shall not exceed 6 ℃/s. Because the PCB and chip may be damaged by excessive heating and cooling rates, which is sometimes invisible to the naked eye. At the same time, different heating temperature and time should be selected for different chips and solder pastes; For the free solder paste, its activity is lower than that of the non-free solder paste. Therefore, the welding temperature should not be too high and the welding time should not be too long to prevent the oxidation of solder particles.

6) During PCB design, the pads of all BGA solder joints on the PCB should be designed to be the same size. If some vias must be designed under the pads, the appropriate PCB manufacturer should also be found to ensure that all pads are the same size, and the solder on the pads is the same and the height is the same. 5 Conclusion With the mainstream development trend of miniaturization of electronic products, the material pin design of BGA packaging will become more and more dense, and the welding difficulty will become more and more difficult. The welding reliability of BGA will always be discussed.

reference

[1] BGA void formation mechanism and its influence on solder joint reliability

[2] IPC - International Association for the Connection of Electronic Industries. IPC - A - 610D Acceptance conditions for printed board assemblies