|Ask the Experts|
September 15, 2020
Channels To Reduce Voids in Large Pads
At our facility we have devices with large pads (D2PAK for example) that have had voids visible through x-ray inspections.
To reduce the voids our eDfM has requested that we add clear channels to the solder paste mask to allow outgassing. However, other facilities do not use these clear channels and our librarians refuse to add these clear channels into the library footprints.
What is the main causes for the voids in these pads?
Is it a solder profile issue, or should these clear channels be defined in the footprints?
|Expert Panel Responses|
There are a number of ways to lower voids in the thermal pad of large devices. One is to provide a path for the gas to escape (clear path). There are numerous papers describing the methods others have found to work well. The patterns reported on include window pane, dots, stripes, X's, and others. But all provide a path for gas to escape.
Some people have reported that slightly higher reflow temperatures and/or longer TALs\ help reduce voids but it is most likely paste dependent.
Another approach is to switch to a low voiding paste. Numerous paste suppliers are offering pastes with fluxes that significantly reduce voids. In one case they report that voids can be reduced to as low as 10%.
I realize that this doesn't help with systems that will not accept changes but it may help your understanding of the issue.
Manager, Process Technology
There is a paper written on this very topic that is available at this link. http://www.indium.com/technical-documents/whitepaper/the-effect-of-thermal-pad-patterning-on-qfn-voiding#paper-request-form
Technical Support Engineer
The expanding gas typically causes voids during the reflow process. These voids can be reduced or in some cases illuminated by making the solder paste stencil cut-out or aperture window pane format.
This "window pane" helps allow gases to escape which will reduce or illuminate void formation. The solder paste is applied prior to smt assembly in small squares over a large pad. This is a common practice for most cm's.
Manufacturing Applications Specialist
Our research has shown that time above liquidus is inversely proportional to voiding area. In some cases, the use of nitrogen, which can allow a longer time above liquidus may be required.
Global Director of Customer Technical Support
Alpha Assembly Solutions
Voiding in large pads is caused by multiple factors. During reflow, the solder paste generates volatile gasses which become trapped in the solder joint creating gaps or voids. The solder paste may not wet the board pads or component leads fully during reflow, which creates gaps or voids. Via hole in pad designs are especially susceptible to voiding. Gasses can rise out of the via holes into the solder joint forming voids.
There are several good methods of mitigating voids. Adding channels in the solder paste print to allow gasses to escape from the solder paste is a good way to reduce voiding. If your circuit board design includes via holes in the pads, we strongly recommend printing solder paste around via holes and not directly over the holes. Gas escape routes should also be added leading from the via holes to the edges of the solder paste print.
The reflow profile should be tuned to work with the solder paste that you are using. Most solder paste manufacturers have specific recommendations on how to adjust the reflow profile to minimize voiding. Some solder pastes have lower voiding potential than others. If you adjust the stencil design and the reflow profile and voiding is still an issue, then we suggest trying a low voiding solder paste. Most solder paste manufacturers have solder pastes that are formulated to reduce voiding.
The Dpak packages are notorious for voids on the solder joints - you can alleviate this problem by changing the stencil aperture design. The right aperture design will reduce and even eliminate the voids that you are experiencing now.
Engineering and Operations Management
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