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

What is the actual root cause of  BGA solder ball cracking? Stress is higher than its bonding-forces.

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

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

BGA solder ball cracked is the certain result of stress higher 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.”

So, 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 want to completely fix the electronic component dropping and solder cracking problems then you must consider the following solutions:

1. Increase the bonding-force of PCBA

This will fix the soldering 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 layer.

The BGA package is commonly found to break from its corners due to the greater distance and stress it sustains. To increase the BGA’s chances of survival during a drop test, design dummy balls on the four corners of the BGA package, and provide bigger pad sizes and more solder volume. This will help to reduce stress concentration on the corners and improve the BGA’s overall durability.

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

The root cause of solder cracking during impact drop tests is often due to the bending of the PCB. Mechanical designers should consider implementing better mechanism structures to prevent PCB bending during high-stress impacts. Another potential solution is to increase the PCB stiffness to make it more robust and less susceptible to bending. By addressing these issues, you can help to reduce the occurrence of solder cracking and improve the overall durability of your electronic components.

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.

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