Why BGA soldering ball always crack(12)? Manufacturing Processes That May Generate Significant Stress


Manufacturing processes that may result in high stress in factories

Inevitably, there are many opportunities to handle and operate PCBs in the manufacturing process of factories, which means there is a chance to apply “stress” to the boards.

Below, Workingbear lists several process in the manufacturing factories where there may be significant stress and precautions for reference:

1. Permanent Deformation of Board Bending Caused by High Temperature during Reflow Soldering

When the reflow temperature exceeds the Tg value of the PCB material, it is easy to cause permanent board bending deformation. Although most boards will gradually return to a flat stage after cooling during reflow, some boards will cause permanent bent and warped due to gravity or uneven distribution of copper foil or other problems.

When the board is bent, its electrical function generally does not have any impact. However, if the production process continues, it is easy to cause additional stress to the board because most assembly or board-end testing processes are assumed to operate on a flat board. Especially when assembling the board into the chassis and locking the screws, the already bent board will be forced to straighten, attempting to restore it to a flat state. This situation is similar to bending a stable board, which will also give additional stress to the formed solder.

To solve this problem, refer back to the explanation of “increasing the resistance of parts to stress”.

Recommended further reading: Causes and prevention methods of board bending and warping

2. Stress Generated during the Testing of Bed-of-nails Fixtures

The bed-of-needle fixtures (ICT, MDA, FVT) for testing of PCB are directly probed at the test points on the board. If the bed-of-needle fixture is poorly designed, it is easy to cause uneven force on the PCBA by the probe, which can cause solder cracking due to bending stress.

If you pay attention, general bed-of-needle fixtures have golden probes and white support rods, which apply a certain amount of stress to the board. Generally, ICT test fixture manufacturers will use strain-stress gauges to measure their stress and make sure strain meet specification before the fixture send to customers, but the vast majority of MDA or FVT fixtures do not perform this stress-strain measurement because it is costly.

3. Stress Generated during PCB De-paneling

In order to improve production efficiency during PCB assembly, it is common to use panelization to produce multiple PCBs at once. Before the PCBs can be installed into their enclosures, they need to be de-paneled into individual boards. The method and operation used to de-panel the PCBs are crucial as improper techniques, such as manually breaking V-cuts or using diagonal cutters to cut stamp holes, can add stress to the board. The worse thing is the stress is difficult to control and can have detrimental effects. The recommended method for de-paneling PCBs is to use a “scoring machine” to cut the V-cuts and a “router machine” to mill the ribs and breakaway of board to avoid applying excessive stress to the board.

Related articles:

  • Design of Stamp Holes for PCB De-Paneling
  • V-Cut De-paneling Machine for PCBs

4. Stress during Whole-System assembly, especially Screw Fastening

During whole-system assembly, the majority of products have screws that fasten the PCBs to the front and back enclosures. From a mechanical perspective, screws are usually fastened in a specific sequence to minimize the stress applied to the board. However, the sequence in which screws are fastened can affect the stress on the board.

Typically, the first screw fastened to the PCB will cause a certain amount of warping, while the second screw will restore most of the original shape. Thus, fastening the first and second screws will have the largest impact on stress. The location of the first screw will also affect the outcome. To determine the best screw fastening sequence, strain-stress gauges are used to measure the stress. If the stress requirements are not met, a stress-balanced screw fastening jig can be designed to solve the problem. The jig would pre-compress the areas where screws need to be fastened to avoid interference and warping caused by the first screw. However, this would add additional time for both the top and bottom jigs. Additionally, the pre-compression screw jig must also be measured for stress too.

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