Some are in favor of a lower peak, while some prefer a higher peak, which has the greater consensus. The ramp to peak/spike: Tent and/or straight ramp, depending on the solder paste company, is currently being used to reduce voiding. For the most part, a "soak" profile is no longer commonly used when having a voiding problem. Soak: There must be a balance between soak that is too slow (avoid oxidation) and a soak that is too fast (entrapping the volatiles). To find the right PCB profile, it is critical to focus on these areas of the thermal profile: Too much heat can cause changes in the LED's luminosity, which can be an expensive error. Some suggestions for reducing voiding in LED assembly is increased temperatures, which can be harmful to the LEDs themselves. Exactly what to change is based on the PCB density and components, finding a balance to fix one area without impacting others. Even when focusing on the thermal profile there are more challenges. The key areas of the thermal profile that can be changed to reduce voiding are the peak temperature, time above liquidus (TAL), the ramp rate, and the soak time. Even if some experimentation is required, the results are available immediately to make decisions quickly. The best news about thermal profiling as a solution, as opposed to the ones above, is that it is an immediate fix. Typically, we see factories invest in one of these ovens for specific products with void-susceptible assemblies. This is an expensive solution and may be difficult to justify. Placement is also a key element in the voiding problem and an important place to look for problems.Īnother solution may be a new type of reflow oven, specifically a vacuum reflow or ultrasonic reflow oven. Trial and error seldom feel like a good solution. However, trying multiple stencils is also costly and time-consuming. Some creative stencil apertures have been made to adjust the amount and spacing of the paste deposition. Thus, changing materials may not be an ideal, or even a possible, solution. Often a low-voiding paste is not the correct solution for all assemblies. Many companies do not want to have multiple solder pastes. Often a contract manufacturer is given specific materials to use when assembling a board, and cannot just change the paste without a qualification process, which is costly and time-consuming.Īlso, while this material may solve the voiding issue for some components, it may cause problems elsewhere on the circuit board. However, making a change to a new solder paste is not a drop-in solution, nor is it always the correct solution. One of the biggest advancements has been in changes to solder paste chemistry. Voiding is attributed to the flux outgassing within the solder joints while the solder is in a molten state. This is why voiding is such an important issue. We deal with the problem of voiding during manufacturing, yet it is the final effect on a product that is key, and sometimes that gets lost. Typically the process window is focused around the solder paste, but often some components cannot accept the temperature tolerances of many lead-free solders. The next challenge is the reflow process window. They range from small PCB assemblies with miniaturization of parts and often a BGA to very large, heavy, double-sided PCBs with ground planes, heat sinks, and densely populated components.Īdded to that is the mixture of circuit board components that cannot be seen with the naked eye, all the way up to tantalum capacitors, BGAs, QFNs, plastic connectors, shields, and much more. Some assemblies seem to ignore any design for manufacturability (DfM) rules. In addition, the variety of PCB assemblies is incredible. With miniaturization and many bottom-terminated components (BTCs) we continue to struggle to find the true causes and the ideal solutions. It continues to be a problem for lead-free assemblies, but the problem has become even more complex. Voiding has been a challenge for quite some time, primarily with BGAs, ever since the shift to lead-free soldering was discussed in the U.S.
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