Reliability analysis and process improvement of BGA welding for conventional electronic components

Date：2022-04-25 14:00:00Views：793

The development of electronic products in the direction of miniaturization, portability, networking and high performance puts forward higher requirements for circuit assembly technology and the number of I / O leads. The volume of the chip is getting smaller and smaller, and there are more and more pins of the chip, which brings difficulties to production and repair. Originally, Quad flatpack QFP was widely used in SMT, and the limit size of package spacing remained at 0.3 mm. The leads with this spacing were easy to bend, deform or break, which had high requirements for SMT assembly process, equipment accuracy and welding materials, and the defect rate of QFP with narrow assembly and fine spacing was up to 6000 ppm, which restricted the wide application.

For BGA devices packaged with ball grid array, since the pins of the chip are distributed on the bottom of the package, changing the pins from the original four sides of the package shell substrate into lead / tin bump pins with area array layout can accommodate more I / O numbers, and replacing the 0.4 and 0.3 mm spacing of QFP with larger pin spacing (such as 1.5 and 1.27 mm), it is easy to use SMT to weld and interconnect with the wiring pins on PCB, Not only can the chip maintain more packaging capacity under the same packaging size as QFP, but also the I / O pin spacing is large, which greatly improves the yield of SMT assembly, and the defect rate is only 0.35 ppm, which is convenient for production and repair. Therefore, BGA has been widely used in the field of electronic product production. In order to improve the quality and reliability of BGA Solder Joints, the defect performance and reliability of BGA Solder Joints were studied.

BGA welding quality and inspection: the welding spot of BGA is under the wafer. After welding, it is difficult to judge the welding quality with naked eyes. In the absence of testing equipment, first check whether the collapse of the outer ring of the chip is consistent, and then point the chip at the light. If each row and column can transmit light, it can be preliminarily judged that there is no continuous welding. However, it is impossible to judge whether there are other defects in the inner solder joint or whether there are holes on the surface of the solder joint. In order to judge the quality of solder joints more clearly, X-ray testing instruments must be used. Commonly used X-ray detection instruments include two-dimensional X-ray direct camera and x-circuit board detector.

The traditional two-dimensional X-ray direct photography equipment is relatively cheap. The disadvantage is that all solder joints on both sides of the PCB board are projected on one photo at the same time. When there are components on both sides of the PCB board at the same position, the shadows formed by these solder will overlap. It is impossible to distinguish which side of the components are and which layer is the problem if there are defects, which can not meet the requirements of accurately determining the welding defects. X-ray circuit board detector is an X-ray tomography equipment specially used to check solder joints. It can not only check BGA, but also check all encapsulated solder joints on PCB. The equipment adopts X-ray tomography, through which the tin ball can be layered to produce the effect of tomography. X-sectional photos can be compared according to the original CAD design data and user set parameters, so as to timely draw the conclusion of whether the welding is qualified or not. The disadvantage is that the price is too expensive.

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. The acceptance standard definition of qualified BGA Solder Joints in 12.2.12 of ipc-a-610c: the solder joints are smooth, round, clear boundary, no cavity, the diameter, volume, gray scale and contrast of all solder joints are the same, the position is aligned, there is no offset or torsion, and there is no solder ball. After the welding is completed, the preferred scheme to judge whether the welding joint is qualified or not is to meet the above requirements, but the standard can be slightly relaxed in the actual inspection. If the position is aligned, the BGA Solder joint is allowed to have an offset of no more than 25% relative to the pad, and the solder ball is not allowed, but the solder ball shall not be greater than 25% of the distance between the two nearest adjacent solder balls.

Common defects of BGA welding common defects of BGA welding: Tin connection, open circuit, missing solder ball, cavity, large solder ball and fuzzy edge of solder joint. Voids are not unique to BGA. Voids can usually be seen visually at the solder joints of surface mount and through-hole insertion components 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 there are holes in these solder joints. It is even considered that holes are beneficial to reliability. Ipc-7095 Committee believes that some holes with very small size and can not be completely eliminated may be beneficial to reliability, but there should be a defined standard for the size.

The solder ball of BGA includes element layer (substrate close to BGA element), pad layer (substrate close to PCB) and intermediate layer. Depending on the situation, voids can occur in any of the three layers. The BGA Solder ball may have holes before welding, so holes will be formed after the reflow process is completed. This may be caused by the introduction of holes in the manufacture of solder balls or the problem of solder paste material coated on the surface of PCB. In addition, the design of PCB is also the main reason for the formation of holes. For example, the via is designed under the pad. During the welding process, the external air enters the molten solder ball through the via. After the welding is completed and cooled, a cavity will be left in the solder ball. The cavity in the pad layer may be due to the volatilization of the flux in the solder paste printed on the pad during reflow soldering, and the gas escapes from the molten solder. After cooling, the cavity is formed.

Poor gold plating on the pad or pollutants on the surface of the pad will cause voids. It is usually found that the position with the most cavity probability is in the component layer, that is, the part between the center of the solder ball and the BGA substrate. This may be because there are air bubbles and volatile flux gas on the BGA pad on the PCB during reflow soldering. When the eutectic solder ball of BGA is integrated with the applied solder paste during reflow soldering, a cavity is formed. If the reflow temperature curve is not long enough in the reflow zone, air bubbles and volatile flux gas have no time to escape, and the molten solder has entered the cooling zone and become solid, forming a cavity. Therefore, the setting of reflow temperature curve is an important reason for the formation of cavity.

Process improvement 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 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 stirred evenly. The viscosity and amount of solder paste must be appropriate to ensure no continuous welding in the process of solder melting.

4) During chip placement, each solder ball on BGA chip must be aligned with each corresponding solder joint on PCB.

5) In the process of reflow soldering, the heating temperature and time of each area should be correctly selected, and the heating speed should be paid attention to at the same time. 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 area, the maximum cooling rate shall not exceed 6 ℃ / s. Because too high heating and cooling rate may damage PCB and chip, this damage is sometimes invisible to the naked eye. At the same time, for different chips and solder pastes, different heating temperature and time should be selected; For non washable solder paste, its activity is lower than that of non washable 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 solder joints of BGA on PCB should be designed to be the same size. If some vias must be designed below the pads, appropriate PCB manufacturers should also be found to ensure that all pads are the same size and have the same amount of solder on the pads and the same height. 5 conclusion with the mainstream development trend of volume miniaturization of electronic products, the material pin design of BGA packaging will be more and more dense, and the welding difficulty will be more and more difficult. The welding reliability of BGA will always be discussed.

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