Defect of the Month with Bob Willis – Underfill Voids

Hello, my name’s Bob Willis and welcome to Defect of the Month. This month I’d like to talk about underfill application and voids in underfills. Underfill is a material which is used in the industry to control and equal out the difference in thermal expansion coefficient of materials. If you have a component which is mounted on a PCB, like a ceramic flip chip or very large area array package you might need underfill. The issue is the difference in expansion rates between the solder joints which make the interconnection, the body of the component and PCB. Putting a material directly underneath the package tends to even out the difference in expansion thus improving the joint reliability. That’s the original concept for underfill materials. Underfills come in different types. One that is a no flow underfill that can be reworked. Alternatively, ones that have a lot of filler which are better in terms of joint reliability but are very difficult to rework

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The first image above shows underfill between a BGA package and the PCB. The second shows a microsection through the component and the PCB with underfill visible. Our third image shows a BGA with adhesive corner bumps.

Other methods of bonding parts to PCBs are generally used today because they are a cheaper solution. These are edge or corner bumps. Some engineers use underfill materials to increase the mechanical strength in terms of drop performance. These materials do not control the expansion rates simply reduce failures during drop testing. If we talk about traditional underfill material what you don’t want are voids because if you’ve got voids, then you’re not getting the same benefits on joint reliability. In addition, you want to try and eliminate voids because of flux residues or contamination that may be present underneath the component.

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The images above show excessive voids or bubbles in the underfill. The second image shows large voids around the solder joints after the component had been mechanically removed from the surface of the PCB

Voids and active flux might potentially create corrosion and you’re not actually achieving a homogeneous coverage of the balls, the PCB, and sealing that gap between the package and the PCB. You may not be able to eliminate voids totally, but you’ve got to think about these as process defect issue. 

Voids can occur during the curing process because of some volatile material which is present in the solder mask surface. That’s one of the reasons why quite often underfill process is directly after reflow, when the board is still theoretically in a dry state. I say theoretically because just reflowing a product won’t get rid of all the moisture. Some companies will bake PCBs right at the very start of the process to eliminate outgassing from the solder mask. But they may apply the underfill very close to the end of the reflow process. This takes advantage of the residual heat in the board assembly to aid the speed of fill 

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The first two images above show testing of a solder mask surface for gassing. The high temperature optically clear oil is used to show if bubbles are present when heating the PCB. The third image shows a thin glass slide on the PCB with underfill and voiding present

Some solder masks if you test them will exhibit gassing. You can just put a droplet of oil on the surface, heat the board up and look underneath the oil. It’s the same as my old outgassing test that I’ve used many times in industry workshops. Just the oil on the surface of the resist will show you if any volatile material is coming out. Hopefully you don’t have any voids. Unfortunately there are not really any international standards to say what voids are acceptable and unacceptable, and that’s something you need to confirm with your product supplier or your contract manufacturer and possibly test products to see whether voiding is an issue to you in your application or not. 

Thank you for listening to Defect of the Month. My name’s Bob Willis, and hopefully you’ll be joining us for more defect of the months videos in the future. Thank you very much.