Why BGA soldering ball always crack(1)? Stress > bonding-force

What is the actual root cause of  BGA ball cracked? Stress is greater than its bonding-forces.

The root cause of BGA solder ball cracked is stress greater than bonding-forces.

Stress is greater than bonding-force is the certain result of BGA solder ball cracked.

BGA solder ball cracked is the certain result of stress greater than bonding-forces.

Working-Bear spent lots of time to prepare this serial article. Don’t just do copy and paste to steal my babies. There will be total 13+1 articles in this blog mentioned the BGA (Ball Grid Array) component drop and solder balls cracked issue. The most concepts that you may know already but here I summary them together systematically and let you read them easily.

“Stress > Bonding-force”

is the root cause of electronic components drop from the PCB and solder crack.

This is not the first time that Working-Bear be challenged the soldering problem from R&D once the NPI run found any components has solder crack. Our RD always came to me and challenged why EMS can’t make thing right to guarantee the solderability. Some RD even ask me need to increase the solder paste volume to improve the soldering strength. It is really enough!

It really makes Working-Bear confusion and cannot understand why people think this is manufacturing issue since the solder crack did not happen at factory site and it happened after impact drop and tumbling reliability test. The solder defect shall come from the stress, but my RD still insist on SMT need to increase the solder paste volume to enhance the soldering strength to fix the solder crack issue.

Working-bear needs to highlight one important concept.

“The main purpose of the solder on the PCB is in order to connect the signals of the electronic components and the PCB. The solder is not to solve the stress effect and PCB deformation problem.”

S0, don’t put the cart before the horse.

Moreover, the soldering strength should be proportional to the soldering area of intermetallic compounds. That means if you like to enhance the soldering strengthen then you will not only increase the soldering volume but also need to increase the area of the soldering Intermetallic compounds. Thinking about if you only apply more solder volume to the same PCB finished, same pad size, same component but the fillet shape on the component foot without change then what will make the soldering strength different? If you like to make sure the solderability between PCB pad and component is good or not then you can make a cross-section on the solder location to check the growth distribution of IMC layer is even will be ok. If you can’t find any problem for IMC then there is nothing SMT factory can do to enhance the soldering strength.

If you like to completely fix the component drop and solder crack issue then you must think about below solutions:

1. Increase the bonding-force of PCBA

This will fix the pad peel off from the PCB.

To enlarge the soldering pad size to increase the pad bonding-force on PCB. You may consider using the Non-Solder-Mask-Defined (NSMD) pad design with plugged via to instead of SMD pad design.

2. Increase the electronic component ability to resist the stress

This will fix the crack on the IMC.

We found The BGA package always broken from its corners. This is because the Corner to corner has the biggest distance and sustain the biggest stress. So, if we can design the dummy balls on the 4 corners of BGA package and give bigger pad size and more solder volume then it will help to increase the BGA survivor chance while drop test.

3. Reduce the stress impact to the PCB and prevent PCB bending/warpage through the mechanism design

The bending of PCB while impact drop test is the root cause to make the solder crack. Mechanical designer shall have better mechanism structure to prevent the PCB bending during stress impact. You may also consider increasing the PCB stiffness to make PCB more robust.

Here comes a concept that Working-bear must emphasize to you again. When the PCB design is done then the soldering bonding-force of the PCB Assembly is basically fixed, but the stress will be a variable factor. Stress will be a critical factor to make solder crack once stress over the bonding-force. Because the source of stress cannot be well controlled in customer site during the product vibration or fall on the ground accidentally, etc. This explains why SMT didn’t change any soldering parameter and material but sometimes you will find components fall or solder crack happening but some of products will not have the soldering issue.  This is because different products may be subjected to different stresses.

Normally, designer shall have the safety factory concept for the product design. Working-bear personal recommend that safety factor must be at least 1.2~1.5 times, which means that product shall able to withdraw the stress higher than the estimated stress of 1.2~1.5 times. You never know how much stress will be applied to our product while used. Besides, the soldering strength of electronic components will gradually deteriorate and become brittle after a period of use. Time will degrade the solderability too.

Related article:
Increase solder paste volume will improve the MLCC capacitor broken?
Why BGA soldering ball always crack(2)? The composition of PCBA bonding-force
Why BGA soldering ball always crack(3)? IMC layer growth is a certain result to form the soldering joints

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