|Ask the Experts|
September 19, 2017
Cause of BGA Solder Joint Bridging
How do you find the root cause of BGA bridging? We are seeing two different bridging areas: one is in the corner and the other is in the center of the array? Is there a way to fine tune the profile to eliminate these defects?
|Expert Panel Responses|
Sure, blame it on the reflow oven and profile - because that's where the bridges happened, right? In reality, you might want to check out the solder paste. First of all, make sure that the profile you are using conforms to what the solder paste manufacturer specifies - you may be suffering from "hot slump". Also, be sure you have the correct solder paste deposit height - yours might be excessive. Anyone assembling with area arrays should have 3-D post-print inspection, at least on an AQL basis. If these actions fail, it might be time to evaluate a new solder paste.
Take a methodical approach. First, look at the board bond pads. Board shops often (too often) make changes in the design and do not communicate this to the end customer. Ensure that the spacing between bond pads is uniform throughout under the BGA Next, take a look at the BGA. Is the spacing between the balls uniform? Deliberate changes (such as an array that is not fully populated) or unintended changes could drive this behavior. Finally, do the same thing with your stencil pattern. If everything looks good, then I would start to suspect out of plane deformation of the BGA (aka, potato chipping) during the reflow process.
CEO & Managing Partner
My initial thoughts are that you are getting warpage or possible popcorning of the packages. Have moisture sensitivity protocols been followed for the BGAs?
Dage Precision Industries
Typical causes of the bridging you describe is excessive top side reflow temperatures. I would suggest you reduce you top-side temperature and increase the subzone temperature. This will reduce the thermal stresses on the component and reduce the chances of the component warping.
Regional Sales Manager
OK International Inc.
An other possible cause of bridging on BGA components that your experts did not talk about concerns the PCB design. When you have vias in pads without filling, the organic medium of the solder paste is trapped in these small "bowls". Flux, solvents, couldn't desorb enough and this generates voids. If you have a picth between balls under 1mm, it could be generate bridging.Jean-Dominique ROUX, ELPACK, France
BGA solder joint bridging may also be because of excessive warpage of BGA device. It essentially occurs when there is some mismatch between the various components that make up a BGA Package (i.e. the substrate, molding compound and silicon chip). Too much warpage creates stress on solder balls and cause Bridging and opens. The warping can be very slight - in this case, the center of the BGA will bow upwards, and the corners down - but it's often enough to cause the corners to move closer to the circuit board surface, putting extra pressure on the corner solder joints to cause bridging.Maninder Singh
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