Solder Voiding Prevention

Issue Description: Commonly caused by gas pockets trapped inside the solder joint.

Voiding Solutions

1) Modify the reflow profile to generate less volatile material and to give more time for the gas to escape.

Increasing soak time will reduce voiding for some solder pastes.

Increasing soak time will drive off more of the low boiling volatiles from the solder paste prior to reflow.

Increasing peak temperature will reduce voiding for some solder pastes.  Increasing peak temperature gives more heat and time above liquidus which allows volatile materials to escape before the solder joint solidifies.

2) Modify the stencil to give gas escape routes under components (QFN thermal pads are common trouble spots).

5-dice, window pane, cross hatch, and radial solder paste deposits all provide gas escape routes.

Clearances can be created around via holes so that solder paste is not pulled into the via hole during reflow.

3) Switch to a solder paste formulation that has a lower tendency to form voids.

Solder paste formulations with lower volatile content obviously will create lower voiding.   For example, WS888 solder paste has lower volatile content than WS889 and will have a reduced tendency to form voids.

No clean solder pastes typically will give lower voiding than water-soluble solder pastes.  No clean solder pastes usually have lower volatile content than water solubles.  No clean solder pastes also have a much lower tendency to absorb moisture from the air.  Moisture absorption into the solder paste results in higher voiding.

Solder alloys with a melting range generate lower voiding than eutectic solder alloys.   The melting range gives a longer time for complete coalescence of the solder powder, which allows gas to escape before the joint becomes fully liquid. This is easily accomplished in lead free solder pastes by using an alloy with some silver like SAC305.

Voiding

Voiding is a hot topic for many electronics manufacturers. Voids in solder joints can create mechanical weaknesses which leads to cracks in the solder joint.

Voids can also interfere with heat transfer away from a component which leads to thermal failure. Many electronics manufacturers are looking for ways to minimize voiding in their solder joints. FCT Assembly is happy to provide solutions to mitigate voiding.

Voiding is caused by a variety of factors. In some cases, multiple factors work together to create voids. The most common factors that cause voiding will be discussed here.

First and foremost, solder paste contributes to voiding. Gasses generated from the solder paste are trapped in the solder joint creating voids. Some solder pastes are prone to generate higher voiding than others.

Newer solder pastes have been formulated with the specific goal of minimizing voiding. Our new no clean, lead-free solder paste called Amp One is a good example of an ultra-low voiding solder paste (Figure 1).

Figure 1:  QFN thermal pad voiding for a typical solder paste as compared to Amp One
Figure 1:  QFN thermal pad voiding for a typical solder paste as compared to Amp One
Figure 2:  Voiding with RSS and RTS reflow profiles
Figure 2:  Voiding with RSS and RTS reflow profiles

The reflow profile must be tuned to work with the solder paste in order to minimize voiding.  Most solder paste manufacturers formulate solder pastes to work with standard ramp-to-spike (RTS) and ramp-soak-spike (RSS) profiles. 

Each solder paste is different and reacts differently to the reflow profile in terms of voiding (Figure 2).

Void area is significantly higher for the RSS profile than for the RTS profile with this solder paste.  We would recommend using an RTS profile with this particular solder paste in order to minimize voiding. 

Again, the reflow profile must be tuned for the solder paste that you are using. 

Figure 3:  Voiding with ENIG and OSP surface finishes
Figure 3:  Voiding with ENIG and OSP surface finishes

Wetting of the solder on the component leads / pads and circuit board pads play a role in voiding.  Poor wetting creates gaps in the solder joint, which by definition are voids. 

Some solder pastes wet and flow better than others which tends to minimize voiding.

Organic Solderability Preservative (OSP) type surface finishes are typically more difficult to wet than Electroless Nickel Immersion Gold (ENIG) surface finishes.  There is a clear difference in voiding between these surface finishes with some solder pastes (Figure 3).

This particular solder paste wets and spreads much better on ENIG than OSP, which leads to lower voiding with the ENIG surface finish. 

Stencil design can have a large impact on voiding especially when the circuit board design lends itself to voiding.  Most electronics assemblers do not have the ability to change the circuit board design but the stencil can be changed to compensate for the circuit board. 

Via in pad design is common for bottom terminated component thermal pads.  Via holes are typically not plugged and therefore contain air or residues from previous processes.  During reflow, gasses from the via holes escape up into the solder joint causing voiding.  It has been shown that printing solder paste around the vias and including gas escape routes in the solder paste print can reduce voiding.

FCT Assembly manufactures solder pastes and stencils to solve problems like voiding in solder joints.  Please let us know how we can help you.