|Ask the Experts|
July 13, 2021
BGA Component Grounding Problem
We have an ongoing concern regarding reliability BGA components. Over the last year, we had boards returned due to a BGA issue. The boards use leaded balls, and leaded solder. All the boards show a resistance to ground at a data point of 3K ohms. A look at "good cards" shows a resistance of 3M ohms. X-rayed extensively and no issue found.
We remove the BGA from the card, and now both the BGA as well as the board show the proper 3M ohms. Each time, after the removal of the BGA, the data point path to ground problem goes away. Essentially we somehow lost the failure signature. Can you offer guidance?
|Expert Panel Responses|
Have you cross sectioned the BGA looking for head in pillow HIP? There are a lot of issues that X-RAY will not show.
President/Senior Technical Consultant
his maybe useful especially if you are using 0.004" thick stencils. I have seen (last six months) many issues of failures and when investigated there is nothing to see under Xray analysis of a few manufacturers BGA's. We found another reflow again with a low solids Rosin flux liberally applied under neath the reflowed device or Rosin flux paste all around the BGA fixed the issue.
I'm pretty sure of the devices I have looked at the BGA balls on a number of pads are miss sized and when using a 0.005" Stencil it's enough to accommodate the miss size whereas the 0.004" is definitely not OK. If you have to use 0.004" Stencils then over print the Paste and allow it to pull onto the pads again making up the gap produced by the smaller balls. Hope it works
Technical Sales Manager
BLT Circuit Services Ltd
If the cause of your high resistance failures is not apparent using microscopy or X-Ray, you will probably need to perform dye & pry and/or cross-section analysis of the interconnects.
Principal Product Engineer
Benchmark Electronics, Inc.
Perform an Dye-n-Pry test on BGAs. It should identify any cracks or separations within the solder joints that could lead to false results. IPC-TM-650 Method 2.4.53 specifies a process for dye-n-pry.
It sounds like what is happening is dendritic growth. Over time, in the presence of moisture and ionic contaminants, a conductive path can form between conductors that are at different potentials. The 3k-ohm resistance is still relatively high for a dendrite, so you probably have very limited dendritic growth.
When you remove the part, you mechanically disturb the microscopic dendrites, and unless significant corrosion has happened, the failure site is then difficult or impossible to find. The "good" resistance of 3M-ohms actually seems very low as well. Normally, I'd expect a resistance above 1 G-ohm.
A resistance of 3M-ohm to ground will result in a small current always passing through that path, which can drive transport of metal ions, and eventually lead to a drop in resistance and failure of the circuit. Some of the most common contributors to this type of failure are:
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