WHY THEY HAPPEN
When assembling printed circuit boards (PCBs) solder paste is printed onto the pads of the PCB using a stencil that is designed to apply the right amount of solder paste to precise locations. The electronic components required will then be precisely placed on the solder pasted pads using precision, hi-speed equipment. When all the parts are on the board, it is inspected and transferred to the conveyor of the reflow oven. In the reflow oven, the PCB is heated using the recommended temperature profile for the solder so it will liquefy, forming an even consistent base under each component. As the paste, heats up and begins to become molten, the flux in the paste turns into a gas. This gas needs to escape while the solder is liquid. If the gas does not discharge, it creates air pockets under the component, and when the solder cools, the gas is trapped. We refer to these air pockets as voids. They may not prevent the PCB from passing inspection and testing, but voids more than 25% may reduce the longevity of the PCB and cause performance issues.
There are a lot of pieces to the surface mount puzzle, and when you begin developing your process, these pieces may not fit neatly together. As you compare the recommendations from the component manufacturer and the solder manufacturer regarding the best stencil design and heat profile, they may not mesh together perfectly. Each suggestion is more like a good starting point, or general guideline. Think of it as if you were baking your favorite recipe. You may start by following the directions precisely, but you find you need to adjust the ingredients, the cooking temperature, or time to get the best results. Your stove and baking pan may not be the same as the recipe writer used, so you have to get creative and use your experience to get the best results. So, it is essential to review all the pieces and select the options that best suit your application. Then tweak the details until you get the right outcome.
THINKING THROUGH THE PROCESS
Surface Finish Of The PCB
When manufacturing PCBs, the copper pads are treated with a protective coating to protect against oxidation. Oxidation will contaminate the copper and may cause voids, which will result in poor connection performance. There are several standard finishes, and each one reacts uniquely with the paste and the recommended heat profile.
Type Of Paste
There are leaded and lead-free pastes. They both are available in “no-clean” options that have minimal flux residue. PCBs containing parts that should not get wet, often use no-clean solder paste. These pastes are also available in different powder sizes (3, 4 & 5) and each one has its’ own best practice procedures for alleviating voids, solder balls, and bridges. Smaller components can benefit from finer powder, like type 4 or 5 since it transfers better during the printing process. However, the use of 4 & 5 may require changes to the stencil thickness and design to maintain adequate solder joint strength.
Solder paste is refrigerated when stored and should be allowed to warm to room temperature just before use. Purchasing high-quality solder paste and following the appropriate handling instructions will play a significant role in void mitigation and joint strength.
Stencil design has become a bit of an art form. Lead-free solder paste has created a lot of challenges in the surface mount world, and it takes some creative solutions to get the best results. The thickness of the stencil, the design, i.e., full coverage, crosshatch, diagonal crosshatch, dots, the size of the pads and components, as well as the type and size of the solder all impact the design and coverage requirements to get the best results and minimize voids.
Heating Profile For The Reflow Oven
Each solder paste has a recommended heating profile for best results. As previously stated, these recommendations are typically a guideline. It may require a couple of tests runs to make sure you get the right outcome.
MAIN TYPES OF VOIDS
The three main categories of voids include Macrovoids, Microvoids, and pinhole voids (See Image 1). Macrovoids can quickly reduce the % of connection between the pad and the part, but the pinhole voids can act as a perforation. Areas with a high concentration of pinholes can be negatively affected by excessive mechanical shock or thermal fluctuations that can cause solder cracking of the joint. Image II represents the target – no voiding detectable!
If you are experiencing unacceptable void issues, refer to similar projects, review the process, and see if you can identify differences. Otherwise, you may need to seek the advice of your solder paste supplier and see what they recommend for your application. Next, is the stencil design suitable for your paste, components, and the board finish? Does the heating profile give the solder ample time to discharge gases? While simultaneously creating acceptable bonds?
We wish you the best as you explore solutions to a common problem.