We search for industry news, so you don't need to.
Ask the Experts  
Questions Index  ■  Submit a Question  ■  Experts Panel  ■  Join the Panel

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?

P.G.

Experts Comments

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.

image
Phil Zarrow
Principal Consultant
ITM Consulting
Mr. Zarrow has been involved with PCB assembly for more than thirty years. He is recognized for his expertise in troubleshooting SMT manufacturing and lead-free implementation. He has extensive hands-on experience with set-up and troubleshooting through-hole and SMT processes throughout the world.

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.

image
Dr. Craig D. Hillman
CEO & Managing Partner
DfR Solutions
Dr. Hillman's specialties include best practices in Design for Reliability, strategies for transitioning to Pb-free, supplier qualification, passive component technology and printed board failure mechanisms.

My initial thoughts are that you are getting warpage or possible popcorning of the packages. Have moisture sensitivity protocols been followed for the BGAs?

image
Dr. David Bernard
Product Manager
Dage Precision Industries
Mr. Bernard has been the X-ray Systems Product Manager at Dage for over 5 years and have been involved in all aspects of x-ray inspection and test for printed circuit board assembly applications. Prior to this, Dr. Bernard was working with radiation measurement instrumentation.

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.

image
Edward Zamborsky
Regional Sales Manager
OK International Inc.
Mr. Zamborsky serves as one of OK's technology advisers to the Product Development group. Ed has authored articles and papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead Free Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.
Reader Comment
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
Reader Comment
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
Submit a Comment

Your Name


Your Email


Company Name


Country


Comments




Authentication

Please type the number displayed into the box. If you receive an error, you may need to refresh the page and resubmit the information.