This article will talk about the composition of the PCBA bonding-force.
The PCBA bonding-force actually contains many items and factors. But working-bear think we can refer to the conclusion of red-dye test to discuss it. The BGA red dye and pry test summary the crack as 5 kinds of defect positions. These locations also can be applied to the PCBA all soldering issues.
1. The bonding-force between Cu Pad (Copper foil) on the component and component substrate (FR4)
The copper foil bonded to the BGA substrate is represented. If there is another type of electronic components, then different substrate material, plating, bonding solution on the soldering foot will present different bonding-force.
2. The bonding-force between BGA ball and Cu pad on component
The IMC layer on the component site is represented.
The BGA package always uses the Nickel (Ni) base of the substrate, so it will come out the IMC of Ni3Sn4. Be noticed that components with different plating material on the soldering foot will result in different IMC composition with different bonding-force.
3. The bonding-force between BGA sphere/ball and Cu pad (Copper foil) on the PCB
The IMC layer grows during SMT is represented.
Different PCB surface finished may present different types of IMC and result in different bonding-force. The Cu base of the PCB will make the IMC of Cu6Sn5 and Ni base of the PCB will make the IMC of Ni3Sn4. Basically the bonding-force of Cu6Sn5 is better than Ni3Sn4. Unfortunately, the Cu6Sn5 will gradually transfer to poor bonding-force of IMC of Cu3Sn as yeas pass.
4. The bonding-force between Cu pad (Copper foil) on PCB and PCB laminate (FR4)
The copper foil bonded on the PCB substrate (FR4) layer is represented.
Recommending NSMD (Non-Solder-Mask-Defined) pad design to improve the copper foil pad on FR4 but NSMD is not good for the solderability on BGA ball. We can have this discussion later article.
Sometimes the humidity attack on the PCB is the key point to result in PCB de-lamination issue due to the moisture will expand quickly during the high temperature reflow process.
5. The bonding-force within solder ball
The void inside the solder ball is represented.
For the other components then the solder paste quality and voids is the key.
Now you know already how many kinds of the bonding-force was included in the PCBA. The next step is to see where is the weakest point then the crack will start from that weakest point while stress applies.
The experience shows there are more than 60% of solder crack show on the IMC layer locate on the PCB side. (Are you still remember what IMC means?). The 2nd one break between the Copper foil pads on PCB and FR4. The 3rd one break between IMC layer between BGA ball and Cu pad on component side. Only few of them break within solder ball or solder.
When people do the analysis for the solder crack for the BGA or component and found there is slightly black color show on broken section people smelled blood and launched a ferocious attack on the SMT factory. Most of the people think this black color must be oxidization happened on the components or PCB soldering surface and it must be the root cause of solder crack. Does this true? We found the color of IMC layer is generally fainter than the flesh-solder since it been exposed at atmosphere quite a long time and most of the solder crack just happen at IMC layer. So, checking the color on the crack section may not give you a correct information.
You still have a question? It is not that the IMC layer growth between the solder alloy and Cu substrate is a necessary condition to form a good solder. Why The solder crack always occurs at the IMC layer?