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Solder joint defect analysis and process improvement

[Abstract]: This paper will combine a number of practical work experience and experience of the BGA solder joint reception standards, defect performance and reliability issues discussed, in particular, have the controversial a defect, the hole is more detailed and thorough analysis, and made some improvement the quality of BGA solder joint process improvement recommendations.

         BGA devices are becoming more widespread, and now a lot of new product design, a large number of application of this device, due to reasons known to all, BGA solder joints after welding quality and reliability of how staff is to make a lot of design and development, assembly, processing of personnel rather for the headache. Unable to use the conventional visual inspection of BGA solder joint quality, debug circuit board found fault, they are often suspected of BGA welding quality problems or the reasons for BGA chip itself, then what kind of BGA solder joints is a qualified What kind of defect will lead to solder joint reliability problems caused by failure or reliability problems? This article will receive the standard BGA solder joints, defective performance and reliability issues discussed, especially for a defect in the disputed conduct a more thorough analysis of hollow.

1BGA Introduction

BGA is a ball grid Chen package devices, which appeared in the 20th century, the early 90s, was due to lead package pin-count devices, more and more smaller and smaller lead spacing, the smallest distance between the device has been achieved 0.3mm (12mil), This assembly is concerned, in terms of manufacturability or reliability of welded components have reached the limit, the chance of error is also growing. Then a new type of ball grid array packages have emerged, compared to the same size of the QFP device, BGA can provide up to several times the number of pins (for BGA solder ball in terms of its chips is equivalent to the following Citation feet) and the pin spacing is relatively large, which is a good thing for the assembly is concerned, you can greatly improve the welding pass rate and a success rate.

PBGA plastic packaging is often used in communications products and consumer products up to a device, it is the ordinary ball component 63n/37Pb, eutectic solder. Sometimes the application of military CBGA ceramic package devices, it is a hot ball of non-10Pb/90 Sn eutectic solder. With the continuous development of BGA devices in the United States and Japan have developed a smaller package, micro-BGA, the chip-scale package size by only less than 20%, generally known as the quilt BGA (microBGA), or CSP, they ball has reached the minimum 0.3mm (12mil), has reached the minimum ball pitch 0.5mm (12mil). In fact, for printed circuit board manufacturing is concerned, in such a small space between the ball produced a very high hole is a difficult task.

         2BGA pull the quality of welding in terms of inspection for BGA solder balls on the chip because of the following, the welding is completed it is difficult to judge the quality of welding. In the absence of inspection equipment cases, the first visual observation of the collapse of the outermost circle of welding is the same, and then chip against the light, if each row of each column can be through the light, it can be concluded that not connected to the welding, and sometimes size bigger solder can also be seen. However, in this way to determine whether the solder joints inside the existence of other defects, or whether the solder joints inside the cavity. To more clearly determine the quality of solder joints, but also must use the X-ray solder joint inspection instrument. The traditional two-dimensional X-ray direct-style camera equipment is cheaper, but its drawback is that both sides of the PCB board that all joints are simultaneously in a photo imaging, for both sides in the same location components, these solder to form may overlap with the shadow together, distinguish which side is the component, if flawed, they need not distinguish Which layer. In this way, can not meet the accurate determination of the requirements of welding defects. We use the HP 5DX circuit board tester is designed to check the solder joints of the X-ray CT scan equipment. Of course, it can not only check the BGA, the circuit board solder joints of all packages that it can check. While the past people thought of this equipment is too expensive, used for solder joint inspection costs are too high, but as more and more extensive application of BGA devices, it has been able to accept such an expensive device.

         3BGA receiving standard solder joints regardless of what kind of screening equipment used to inspect the quality of solder joints to determine eligibility must be evidence-based. IPC-A-610C 12.2.12 devoted to receiving the standard BGA solder joints are defined. Optimization of the BGA solder joints requirement is smooth, round, clear boundaries, no hole, all the solder joints of the diameter, volume, gray scale and contrast are the same, position alignment, without offset or reverse, without solder balls. Welding is completed, the optimal standard is the goal, but as a qualifying criterion of solder joints, but also requires a slightly more relaxed than the above standards. Such as the location of alignment, allowing BGA solder joint relative to the pads not more than 25% of the offset for the solder ball is not absolutely not allowed to exist, but can not be larger than the adjacent solder ball the past two ball pitch 25 %.

         Typical BGA solder joint defects 4BGA Typical defects include: Lian welding, open, ball lost, big hole, large solder balls and solder the edge of ambiguity.

5 a defect in the disputed

An empty one now remaining in the controversial issue concerns the BGA in the empty reception standards. The problem is not unique to empty BGA. In the through-hole and surface mount and through-hole components, solder joints are generally allowed to use visual inspection to see empty, instead of X-rays. In the BGA, because all of the solder joints hidden in the package below, only the use of X-rays in order to check these joints. Then empty the reliability of BGA necessarily have a negative influence? Not necessarily. Some people even say that the reliability is good for the hole. IPC-7095 standard "The design and realization of BGA assembly process" in detail the realization of the design and assembly of BGA and technology. IPC-7095 the Committee considers that some dimensions are very small, can not completely eliminate the empty may be beneficial for the reliability, but what size should have a defined standard.

          5.1 empty position and the reasons for the formation of the solder joints in BGA inspection can find an empty place in what it? BGA's solder balls can be divided into three layers, one component layer (near the BGA substrate components), one pad layer (near the PCB substrate), another one is the bump in the middle layer. Depending on the circumstances, empty can occur in these three layers in any one layer. What is the time to empty come from? BGA solder ball may itself before welding with a hollow, so that after the reflow process is complete, the formation of a hole. This may be due to ball making process on the introduction of a hole, or the PCB surface coating of solder paste materials, the issue causing the problem. Another circuit board design is also a major cause of the formation of hollow. For example, the through-hole design in the pad below, in the welding process, the outside air through the hole into the molten solder ball melting state, the welding in the ball after the completion of cooling would be left empty. Occur in the empty pad layer may be due to the printing of solder paste pad above the flux in the reflow soldering process of evaporation, the gas from the molten solder in the escape depth of the cooling after the formation of a hole. Pad pad surface coatings are poor or contamination may occur in the pad layer of empty reasons. Probability of the most commonly found in an empty location is at the component level, which is central to the BGA solder balls between the part of the substrate. This may be due to the above BGA pads on PCB reflow soldering process, there are air bubbles and volatile gas flux, when the BGA solder balls with the eutectic solder paste applied in the reflow soldering process of melting formed when a body cavity. If the reflow temperature profile in the reflow zone long enough, the air bubbles and the flux of volatile gas too late to escape, melting the solder melting speculation has entered the cooling zone into a solid state, they form a hollow. Therefore, the reflow temperature profile is the reason for the formation of hollow species. Eutectic solder of the BGA most readily 63n/37Pb hole, made into the illegal 10Sn/90Pb eutectic melting point solder balls of the BGA, the melting point of 302 , in general largely empty, it is because the re-melted in the solder paste flow welding process on the BGA solder balls do not melt.

          5.2 empty empty reception standards in the presence of gas may be generated during thermal cycling contraction and expansion of the stress will become a hollow where there is stress concentration points, and may cause stress cracks to become the root cause. However, the existence of empty reduces the solder balls due to excessive inter-Shen, also reduces the mechanical stress on the solder ball. But also reduce the number of empty depending on the specific size, location, shape and other factors. IPC-7095 set forth in the empty reception / rejection criteria have two main considerations: the location and size is empty. Empty no matter what position there is a solder ball in the middle or in the pad layer or component layers, depending on the size and number of empty will cause quality and reliability of different effects. Ball inside the ball allows the existence of small size. Empty space, occupied space and ball ratio can be calculated as follows: for example, solder ball diameter hole diameter of 50%, then the empty area is occupied 25% of solder ball area. IPC standards required to receive the standard as follows: the empty pad layer can not be greater than 10% of the ball area that is empty in diameter can not exceed 30% of the solder ball diameter. When the pad layer of hollow ball size area of more than 25% is regarded as a defect, then there would be an empty solder joint reliability of mechanical or electrical caused a hidden danger. In the empty area of pad layer 10% ~ 25% of the bump area, they should focus on improving the process, to eliminate or reduce empty.

6 Conclusion

BGA defect reduction process improvement suggestions eutectic solder BGA solder joints during the welding process to form, PCB board coated with solder paste and components of the hides of the solder balls to melt as a whole, this process is divided into two stages of collapse . The first phase of the collapse is the PCB board, solder paste first melts, components fall down, and the second stage is the element itself is also melting solder balls with the PCB board of the molten solder paste melting into one, again, the collapse of solder balls down, forming a flat circular solder joints. To form a perfect solder joints, should pay attention to the following aspects:

(1) The use of fresh solder paste, solder paste stirring to ensure uniform coating paste the location of accurate and precise component placement location.

(2) For the PBGA plastic package before soldering to 100 drying 6-8 hours, with nitrogen conditions, if the better.

(3) The return of the temperature curve is a very important factor. In the welding process, to ensure that welding the natural curve of the transition, so that uniform heating device, especially in the welding zone, to ensure that all joints fully melted. Otherwise it will lack the formation of cold solder joints due to temperature, solder joint surface roughness, or second collapse phase is not fully melted, PCB surface of the solder paste and components itself, the middle of the cracks, causing cold solder joint or open solder.

(4) The coating of the solder paste volume must be appropriate, the viscosity of solder paste should serve the role of the device is temporarily fixed, but also ensure that the solder does not melt during welding with welding. Usually produce BGA template, BGA solder joint pad size of hole size is usually 70 ~ 80%, the template is usually the thickness of 0.15mm (6mil).

(5) The design of BGA pads on PCB Be sure to pad all the solder joints are designed as large, if some people had also must be designed to pad the following should also be to find a suitable PCB factory, in the the location of the drilling pad, but not because it can not be so small over-drilling holes on the pad to change without authorization will be big, so big after soldering pads on the pad and the small amount of tin is not as much as a high degree of inconsistency also cause cold solder joint or open circuit.

(6) In addition, it asserted that it is on the PCB solder mask when making the problem. As a result of welding solder mask unqualified failure is already a lot, so check first before soldering BGA solder pad around the film is eligible, the welding surface around the through-hole pad is also necessary to prevent solder coating film. If you create at the time of the PCB solder mask added to the other side of useless. Canadian solder mask objective is to avoid welding of air coming from below the formation of hollow, but also prevent the outflow of solder from the hole. If the printed solder paste shall not rework, then there will be no excess solder does not affect the welding quality, because the through-hole plating hole itself, but if too much solder or produce sharp pull, solder balls of the type of problems, it will short-circuit the hidden dangers left behind, it was called "virtual short" defects. BGA rework is a measure of last resort, although there are likely to repair a weld failed BGA chips, but repair a BGA have to spend a longer period of time, but also must have the right to ball and repairing tool for precise alignment. Ball's approach has been presented many papers in, but the actual operation of the ball when planting is usually not very high success rate. Sometimes in order to rework a BGA to spend at least half a day's time, the resulting waste of resources is obvious. Even if repaired were re-welded up, the chip has been suffering from at least four times the return cycle, which would definitely affect the reliability of welding, such as fatigue and creep failure will be accelerated. In short, prior to soldering BGA fully prepared, it was entirely possible to achieve a high pass rate, increasing a success. To minimize or eliminate the defects, no rework, this is our goal

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