Preventing plating and soldering voids is about testing new manufacturing processes and analyzing the results. Plating and soldering voids usually have an identifiable cause, such as the type of solder paste or drill bit used in the manufacturing process. There are several key strategies that PCB manufacturers can use to identify and resolve these common causes of void formation.
Adjust reflow profile
One of the best ways to prevent soldering voids is to adjust key zones of the reflow profile. The amount of time given to different stages can increase or decrease the likelihood of voids forming. Understanding ideal reflow profile characteristics is vital to successful void prevention.
First, take a look at the current settings for the preheat time. Try raising the preheat temperature or extending the time allowed for the preheat zone of the reflow profile. Solder voids can form due to inadequate heat in the preheat zone, so use these tactics to address that root cause.
The soak zone is also a common culprit for soldering voids. A short soak time may not allow all components and areas of the board to reach the necessary temperature. Try allowing some extra time for this zone of the reflow profile.
Apply less flux
Excessive flux worsens an additional problem that commonly causes soldering voids: flux outgassing. If the flux doesn’t have enough time to outgas, that extra gas can become trapped, creating voids.
When too much flux is applied to the PCB, it extends the amount of time needed for the flux to fully outgas. Unless you incorporate additional outgassing time, the extra flux will lead to soldering voids.
While adding more outgassing time can address this issue, it is more efficient to stick to the minimum amount of flux necessary. This saves energy and resources and leads to cleaner joints.
Use only sharp, precise drill bits
The most common cause of plating voids is poor through-hole drilling. A dull drill bit or poor drill precision can increase the likelihood of debris created in the drilling process. When this debris sticks to the PCB, it creates blank spaces where copper plating doesn’t get applied. This damages conductivity, quality and reliability.
Manufacturers can address this issue by using only sharp and precise drill bits. Set a consistent schedule for sharpening or replacing drill bits, such as once every quarter. This regular maintenance will ensure consistent through-hole drilling quality and minimize the likelihood of debris.
Try different stencil designs
The stencil design used in the reflow process can help or hinder soldering void prevention. Unfortunately, there’s no one-size-fits-all solution to stencil design selection. Some designs work better with different solder pastes, flux or PCB types. It may take some trial and error to find the best option for a specific board type.
Successfully finding the right stencil design requires good testing processes. Manufacturers must find a way to measure and analyze the impact of a stencil design on void generation.
One reliable approach is to create a batch of PCBs with a certain stencil design, then conduct a thorough inspection of them afterward. Do this with several different stencils. The inspections should reveal which stencil designs have the lowest average number of solder voids.
A key tool for the inspection process is an X-ray machine. X-rays are one of the only methods for finding soldering voids and are particularly useful when working with small, tightly packed PCBs. Having an X-ray machine handy will make the inspection process significantly easier and more efficient.
Reduce drill speed
In addition to drill-bit sharpness, drill speed can have a big impact on plating quality. If the drill speed is too high, it can reduce precision and increase the likelihood of debris forming. High drill speeds can even increase the risk of fractures in the PCB, threatening structural integrity.
If plating voids are still common after sharpening or replacing the drill bit, try reducing the drill speed. A slower speed allows a bit more time for precise, clean through-holes to form.
Remember that conventional manufacturing methods aren’t the only option on the table today. If efficiency is the concern driving high drill speeds, 3D printing may be a good alternative. 3D-printed PCBs can be manufactured more efficiently than conventional methods but with the same or higher level of precision. Opting for 3D-printed PCBs may remove the need to drill through-holes at all.
Stick to high-quality solder paste
It’s natural to search for ways to save money in the PCB manufacturing process. Unfortunately, buying cheap or low-quality solder paste can increase the likelihood of soldering void formation.
The chemical properties of different solder paste varieties can impact their performance and the way they interact with the PCB during the reflow process. For example, using paste that does not contain lead may shrink during the cooling process.
Choosing high-quality solder paste requires you to understand the needs of the PCB and stencil being used. A thicker solder paste will have a difficult time penetrating a stencil with a low aperture size.
It may be useful to test out different solder pastes while also testing different stencils. Focus on matching up the solder paste with the stencil using the five-ball rule for stencil aperture sizing. This rule states that manufacturers should use a stencil with the smallest aperture necessary to fit five balls of solder paste. This concept simplifies the process of creating different paste-stencil configurations to test out.
Reduce solder paste oxidation
Solder paste oxidation often occurs when there is too much air or moisture in the manufacturing environment. Oxidation itself can increase the likelihood of voids forming and also hint at excess air or moisture that further increases the risk of voids. Addressing and reducing oxidation can help prevent void formation and improve PCB quality.
Start by checking the type of solder paste in use. Water-soluble solder pastes are particularly vulnerable to oxidation. Additionally, inadequate flux can increase the risk of oxidation. Of course, too much flux is also a problem, so manufacturers have to find a balance. However, if oxidation is occurring, increasing the amount of flux often resolves the issue.
There are many steps that PCB manufacturers can take to prevent plating and soldering voids on their electronics. Voids compromise reliability, performance and quality. Fortunately, minimizing the likelihood of void formation can be as simple as changing the solder paste or using a new stencil design.
Using a test-inspection–analysis approach, any manufacturer can find and resolve the root causes of voids in their reflow and plating processes.