Analysis of the causes of "virtual welding"
From: Author:Mark Hardy Publish time:2021-09-09 14:25 Clicks:0
2.2 Analysis of the causes of "virtual welding"
Analyzing the problems that occurred in actual production, it is believed that the possible reasons for the formation of "virtual welding" are as follows.
2.2.1 Oxidation of solder ball and pad surface
If the solder balls of the device are oxidized or the PCB pads are oxidized, it is difficult for the Korean materials to form a firm metallurgical bond with the pads, and thus cannot provide continuous and reliable electrical performance, which is a phenomenon of "virtual soldering".
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2.2.2 Solder joint cracks
If the BGA solder joint has cracks at the interface, resulting in mechanical and electrical performance failure, we also call it "virtual solder." BGA solder joint cracks are mainly caused by the mismatch of the base expansion coefficient of the PCB substrate and the components (the CTE of FR4 is 18ppm/℃, and the CTE of the silicon chip is 2.8ppm/℃), and the residual stress in the solder joints. Studies have shown that most of the cracks in BGA solder joints (whether SnPb or SnAgCu solder joints) occur between the solder balls and the device substrate, that is, on the package side, and the cracks are very close to the intermetallic compound on the package side. The software simulation and test results are consistent. Personally, this conclusion exposes the quality problems of the device itself to a certain extent. Figures 3 and 4 are the metallographic analysis diagram and optical inspection diagram of the BGA solder joints, and the crack appears on the upper end of the device.
As long as the bottom of the crack does not penetrate deep into the solder joint to affect the electrical and mechanical properties, it can be judged as qualified. But if there are cracks in the solder joints, it may not affect the electrical performance of the whole machine temporarily, but the cracks further expand under the high and low temperature cycle or impact load and the solder joints are disconnected, which will cause the whole machine to fail. Therefore, in actual production, Mr. Zhang is looking for that, especially for military products, BGA solder joints are not allowed to appear cracks.
2.2.3 Cold solder joints
In the reflow stage, if the solder stays above the liquidus temperature for too short a time, the solder and the solder balls will not be fully fused together and then enter the cooling zone, which will result in cold solder joints, which have rough surfaces and poor long-term reliability. , It is easy to cause the failure of the solder joints and form a "virtual solder".
2.2.4 Other
Mainly reflected in the printed board design and printed board manufacturing. If the quality of the solder mask between the BGA pad and the via is not good enough or damaged, or the via is designed below the pad, the solder paste will easily flow into the via during heating, making the solder joint at the solder joint. The amount of paste is reduced, and the solder balls of the entire BGA device are not coplanar, which becomes a hidden danger of "virtual soldering".
3. Improvement measures
3.1 Storage and pretreatment of the device
BGA device is a highly humidity sensitive device (especially PBGA), so BGA must be stored under constant temperature and dry conditions. Generally speaking, the ideal storage environment for BGA is 20℃~25℃, and the humidity is less than 10%RH. The table shows the humidity-sensitive classification of components. It indicates that during the assembly process, once the sealed package is opened, the components must be used for the corresponding time of installation and welding. Generally speaking, BGA belongs to level 5 or above.
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