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BGA solder joint cracking has always been a nightmare for electronic assembly (industry) manufacturer, and there are many reasons that are beyond the control of assembly manufacturer, but the final result is often required to be borne by assembly manufacturer.
There are many reasons for BGA solder joint failure, but the main reason comes from PCB deformation, resulting in solder joints to be break under bending stress. PCB deformation usually comes from two reasons:
The rapid rise and fall in temperature caused by high temperature reflow soldering, resulting in thermal expansion and contraction, coupled with the weight of the PCB and electronic components, as well as uneven layout of copper foil on the PCB, making the deformation of the PCB more severe.
Product drop impact or bending deformation causes PCB deformation.
After understand that the cracking of BGA solder balls is closely related to PCB bending deformation, we can now talk about how to strengthen the BGA and prevent cracking from the design end. There are two directions to consider: the first method is to reduce the impact of stress, and the second method is to enhance the BGA’s resistance to stress.
Here are several methods to strengthen BGA and prevent cracking:
1. Enhance PCB deformation resistance
There are several methods to increase the circuit board’s resistance to deformation:
Increase the thickness of the PCB. If possible, it is recommended to use a PCB with a thickness of 1.6mm or more. If the thickness of 0.8mm, 1.0mm, or 1.2mm still need to be used, it is suggested to use reflow carrier to support PCB and strengthen PCB and prevent it from deformation during reflow.
Use high-Tg PCB materials. High-Tg means high rigidity, but the price of PCB will also increase, so it is necessary to make trade-offs.
Add reinforcement ribs around the BGA. If space permits, consider adding supporting metal frames around the BGA to enhance its ability to resist stress, similar to building a house.
Potting epoxy resin on the PCB. It is also possible to consider dispense glue around the BGA or on the back side of the corresponding PCB to strengthen its ability to resist stress.
2. Reduce PCB deformation
In general, Once a PCB is assembled into a casing, it should be protected by the casing. However, because today’s products are getting thinner, especially handheld devices, PCB deformation often occurs due to external bending or impact caused by drops.
To reduce the deformation caused by external forces on the PCB, the following methods can be used:
Increase the buffering design of the structure on the PCB. For example, design some buffering materials, so that even if the casing is deformed, the internal PCB can still be protected from external stress. However, the service life and capacity of the buffering materials must be considered.
Add screws or positioning fixed structures around the BGA. If our goal is only to protect the BGA, then we can forcibly fix the structure near the BGA to prevent deformation near the BGA.
Strengthen the casing strength to prevent its deformation from affecting the internal PCB.
3. Enhance the fastness of BGA
dispense “Underfill” glue at the bottom of the BGA.
Enlarge the size of the BGA solder pads on the PCB. This will enhance pad bonding strength on the PCB. But this also makes wiring on the PCB more difficult because the space between the balls for signal routing is reduced.
※Note: The SMD and NSMD pad designs actually have their respective advantages and disadvantages. Adopting SMD can enhance the bonding strength between the solder pads and the PCB, but it is not conducive to the solder joint strength of the solder balls. Adopting NSMD will enhance the solder joint strength, but it is not conducive to the bonding strength between the solder pads and the PCB.
Use Vias-in-pad (VIP) design. However, the via holes on the solder pads must be filled with copper electroplating to prevent soldering voids during reflow, which can cause the solder balls to break in the middle. This is similar to the idea of anchoring a house with ground screws to prevent seismic damage.
Increase the amount of solder volume. It is noting help to strengthen the solderability for BGA ball. Increasing the solder volume is in order to reduce the HIP/HoP of soldering defect. However, it should be ensured that this increase does not result in soldering short.
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