How to reduce voids under BGA and QFN solder joints
Voids are gas pockets trapped in a solder joint that lower thermal conductivity and reliability. This guide shows how to reduce voids under BGA and QFN through paste choice, a clean preheat profile and vacuum reflow, and how to judge the result against IPC-7095 and IPC-A-610.
View soldering guideHow do voids form under BGA and QFN?
Voids are gas bubbles trapped in the molten solder during reflow. Most of the gas comes from flux outgassing in the solder paste: solvents and activators evaporate, and under large areas such as BGA balls or the QFN centre pad the gas cannot escape in time.
The central thermal pad of QFN and LGA packages is especially critical. The large continuous solder area acts like a trap, so voids there often reach 40 % or more of the area when the profile and paste deposit are not adapted.
- Flux outgassing is the main source of the trapped gas.
- Large pad areas (BGA ball, QFN pad) make escape harder.
- Oxidation on pad or ball hinders wetting and holds gas in place.
- Heating too fast drives solvents out only once the solder is liquid.
What role do paste and preheat play?
The solder paste is the first lever. Type 4 and Type 5 pastes with finer powder and flux specifically tuned for low voiding outgas more gradually. Equally important is a long enough soak phase between roughly 150 and 180 °C so solvents leave before the solder melts.
- A nitrogen atmosphere improves wetting and reduces oxidation.
- Fresh, properly stirred paste avoids moisture uptake.
- Do not overshoot the peak: too hot burns off the flux too early.
When is vacuum reflow worth it?
In vacuum reflow the chamber is briefly evacuated to a few millibar up to around 100 mbar while the solder is liquid. The trapped gas bubbles expand and rise out of the melt before the solder solidifies. This can cut void content by 50 to 80 %, sometimes to residual values below 2 %.
Vacuum pays off mainly for power semiconductors, large QFN centre pads and safety-relevant BGA where profile tuning alone cannot reach the required limits. The vacuum is applied deliberately in the liquid state and released before solidification.
How do you assess voids to IPC?
Assessment uses X-ray inspection judged against IPC criteria. IPC-7095 covers void calculation for BGA and IPC-A-610 defines acceptance criteria. A common limit is 25 % void area per individual BGA ball, projected in top-down view.
- 2D X-ray for the top-down void area per ball.
- Check position within the ball: interface voids are more critical than central ones.
- Link results to the profile and paste batch and archive them.
Frequently asked questions
At what void level is a BGA joint rejected?
The common guide value under IPC-7095 and IPC-A-610 is a 25 % projected void area limit per individual ball. For critical power applications many makers agree stricter project-specific values.
Does a better paste alone reduce voids enough?
Void-optimised Type 4 or Type 5 pastes help but rarely suffice on their own. Only combined with a clean soak profile and, on large pads, vacuum reflow do you reach consistently low values.
How much does vacuum reflow cut voids?
In practice reductions of 50 to 80 % are common, often to residual values below 2 %. The vacuum is applied while the solder is liquid so the gas bubbles rise before solidification.
How are voids measured?
The standard is X-ray inspection, usually 2D top-down. The void area is compared to the ball or pad area and assessed against IPC.
Get voids safely under the IPC limit?
From void-optimised solder paste and stencils to reflow and vacuum systems, we supply the right equipment for BGA and QFN.
IPC-oriented
Assessment to IPC-7095 and IPC-A-610.
Vacuum know-how
Reflow and vacuum profiles for low voids.
Right paste
Void-optimised Type 4 and Type 5 solder pastes.
Expert advice
Soldering specialists support your process design.