Voiding in solder joints is a concern for various reasons. Voids can lead to cracking or mechanical failures in solder joints. It can interfere with heat transfer leading to over-heating related failures. It can also create noise or interference in electrical signals.
FCT Assembly has many solutions to reduce voiding which can be minimized through the ultra-low voiding solder pastes like Amp One. Voiding can also be reduced through stencil design and reflow profile modifications.
Using Solder Paste to Reduce Voiding
One major contributor to voiding in solder joints is solder paste. Solder pastes contain ingredients that volatilize during reflow. These volatile materials become gas bubbles that can be trapped in the solder joint. Different solder pastes also wet and spread differently. Incomplete wetting or spreading on the board pad and component surfaces leads to voiding. FCT Assembly has formulated solder pastes with the specific intention of reducing voiding potential. Our ultra low voiding solder pastes have reduced volatile content and optimized wetting. Contact FCT Assembly if you would like to test samples of our ultra low voiding solder paste.
Stencil Design to Reduce Voiding
Modification of the stencil design is one possible solution to voiding. Our partner, BlueRing Stencils can offer specific stencil designs to minimize voiding. Bottom terminated components require the solder paste deposit to be broken up with a crosshatch pattern. This gives channels for gasses to escape from the solder joint before the solder freezes. When using via-in-pad designs, solder paste can be printed around the via holes rather than directly over them. This prevents solder paste from flowing down into the via holes, and minimizes voiding. Contact BlueRing Stencils for assistance reducing voiding with stencil design.
Reflow Profile for Voiding
The reflow profile can contribute to voiding in solder joints. If the ramp rate is too high then volatile materials may not be completely driven off and can lead to voiding. High ramp rates or short reflow profiles can lead to incomplete wetting or spread of the solder paste. High peak temperatures or long reflow profiles can cause less than optimal wetting, and the flux may generate additional gases which can lead to voiding. The reflow profile must be tuned to work with the solder paste in order to minimize voiding. Contact FCT Assembly for assistance with reflow profiling to minimize voiding.
Tombstoning is a defect that typically affects smaller chip type components (0402’s or 0201’s). These chip components flip upwards at one end breaking the connection between the component and the board pad. This typically occurs when the solder paste on one end of the component melts before the other end of the component. As the molten solder coalesces, enough force is generated to pull the component up and out of the solder paste which has not melted. Anti-tombstoning solder pastes widen the melting range which helps the solder paste at both ends of the component to melt simultaneously.
Stencil modifications can also be used to minimize tombstoning. Removing some solder paste underneath the ends of components in the form of a U-shape or inverted home plate helps to minimize tombstoning. Adding or lengthening the soak in the reflow profile equalizes the temperatures across the circuit board, which helps the solder paste to melt simultaneously at both ends of the component. Contact FCT Assembly and BlueRing Stencils for solutions to tombstoning.