PCBA Tech - How does PCBA judge the cause of BGA cracking from red ink?

The following is the introduction of PCBA to judge the reason of BGA cracking from red ink:

When BGA failure occurs, use the "Red Dye" test to analyze the problem and find that there is a crack in the solder ball. Do you know how to judge the true cause? Or is the red ink only telling us that there is no tin ball cracking, and the others can only ask for their blessings?

Generally, BGA solder ball cracking can be roughly summarized as HIP (Head-In-Pillow) or HoP (Head-On-Pillow) and NOW (Non-Wet-Open) and post-soldering stress, but each phenomenon may have compound causes. In fact, if you want to know what kind of bad phenomenon BGA solder ball cracking is, the best way is to do slices and elemental analysis, so that there is more objective evidence to prove the true cause. However, before slicing, you must first confirm which solder balls are faulty through the method of store testing, so that the solder balls can be sliced.

It is recommended to read first:

Introduction Use the Red Dye Penetration Test to check whether the BGA solder is broken

To be honest, Shenzhen Honglijie personally does not like to use "red ink" to analyze the problem of BGA solder ball cracking, because the red ink test is easy to make mistakes, because accidental operation may destroy the original evidence and Phenomenon, and it is not easy to judge the true cause. But red ink is indeed the cheapest test method at present, and as long as you have an oven, you can do it yourself if you buy red ink, but red ink is a destructive test. If you only have one sample, it is recommended that you pass it first. X-Ray checks whether there is empty soldering and uses the "fiber camera" to check the solder balls on the edge of the BGA for HIP or NWO.

If you have enough bad samples, not just one, Shenzhen Gracetech recommends using red ink for preliminary verification before slicing, because the red ink test generally only shows whether the solder ball is cracked, and the tin Distribution of ball cracking.

Strictly speaking, after the red ink test, it should be placed under a high-power microscope to observe the cracked cross-section phenomenon. It is necessary to know whether the cross-section is a smooth arc cross-section or a rough cross-section to determine whether it is a HIP/HoP or NWO SMT process or It is the impact damage caused by the external force caused by the PCBA assembly or the finished product falling on the ground afterwards.

Therefore, the following phenomena should be recorded during the red ink test:

red-dye-record

▪ Record the area of each solder ball dyed red by red ink.

▪ Observe and record the crack of the solder ball between the solder ball and the BGA carrier board (continue to check whether the pad of the BGA carrier board is peeled off, whether the red ink enters the peeling point of the solder pad), between the solder ball and the PCB (continue to check Whether the PCB pad is peeled off, whether the red ink enters the peeling point of the solder pad, or is broken in the middle of the solder ball (check the cross-sectional shape).

▪ Record the appearance and shape, smoothness or roughness of all fractured surfaces.

Judgment of the result of the red ink test (there may be other compliance reasons):

▪ If the fracture occurs between the solder ball and the BGA carrier board, continue to check whether the pad of the BGA carrier board is peeled off, whether the red ink enters the peeling point of the solder pad, if there is, this may be related to the problem may come from the BGA carrier The quality of the board or the pad strength of the BGA carrier board is not enough to bear the responsibility for the problem caused by external stress.

▪ If the crack occurs between the solder ball and the PCB, continue to check whether the PCB pad is peeled off, whether the red ink has entered the peeling point of the solder pad, if not, it may be a NWO problem, but continue to observe the shape and shape of the cross section. Roughness, if the solder pad is peeled off and the red ink enters the peeled area, the problem may come from the quality of the PCB manufacturer or the strength of the PCB itself is insufficient to bear the cracking caused by external stress.

▪ If the fracture occurs in the middle of the BGA solder ball, it is necessary to continue to observe the cross-section of the arc surface, smooth surface, rough surface, flat surface, if it is a smooth arc surface, it may be HIP, on the contrary, it is more external Cracking caused by stress.

If it is NWO and HIP, it is biased towards process problems. This type of problem is usually caused by the deformation of the PCB board or the BGA carrier board when the reflow temperature is high, but the amount of deformation is also related to the design of the product. The copper foil wiring on the PCB is uneven or When it is too thin, it is easier to cause deformation. The second is the oxidation of the solder plating.

Extended reading: Causes of plate bending and plate warping and methods to prevent

The cracking of BGA solder ball caused by external stress generally has the following possibilities:

▪ The board level test is caused.

The general board step test will use a test fixture such as a needle bed. If the stress distribution on the needle bed fixture is uneven, the BGA solder ball may be broken. It can be checked with a stress-strain analyzer.

▪ PCBA finished product assembly.

Some poorly designed finished PCBA assembly operations may be bent to the circuit board. This may be caused by locking screws, using hooks for positioning, and the mechanism of the finished chassis supporting the circuit board not being designed on the same level... You can use a stress-strain analyzer to check.

▪ The bending stress of the plate caused by the impact of the finished product falling to the ground due to careless use at the client.

This can also be calculated by using a stress-strain analyzer to simulate the amount of deformation when falling.

▪ The thermal expansion and contraction deformation caused by the environmental temperature change.

This reason is relatively rare, because it should be able to be caught in the research and development stage, unless the environmental test is not thoroughly performed.

Please remember that if the solder ball is cracked by stress, it will start to crack from its weakest place first. If the reflow soldering of SMT is not soldered well, it should break the solder between the BGA ball and the circuit board. Between the pads, the problem of material oxidation is not considered for the time being. If the design of the solder pad is too small to withstand too much impact force, it will cause the solder pad on the circuit board to be pulled down. In fact, this phenomenon may also be caused by the PCB manufacturer poor pressing.