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Thermal profiling optimizes printed circuit board assembly

Posted on Apr 01, 2008

Thermal profiling is a critical function in printed circuit board assembly. This article describes thermal profiling hardware and process management software solutions as used at EMS provider Axiom Electronics LLC to meet the thermal profiling challenges of today’s microelectronics environment. Reflow challenges include thermal inequalities of high layer count circuit boards that have uneven mass distribution, components of varying sizes, micro BGAs, high ball-count BGAs, LGAs, etc. These easy to use thermal profiling tools help to improve yields by providing a stable repeatable process while saving time and providing necessary supporting documentation.

Today's challenges in thermal profiling

Although the electronics industry is often regarded as mature, new challenges continually occur. Some of these challenges, such as smaller parts and lead-free assemblies, require further development or modification of equipment, or changes in the control software.

Component packages are available in more sizes, from 01005s to 50mm2 BGAs, and are being used side-by-side on the same circuit board. This can lead to problems when assemblies are thermally profiled for reflow. Layout, component size, package styles and thermal mass of the selected devices add to the challenge. Thermal issues include different tolerated maximum temperatures, thermally sensitive components and different formulations of solder and flux.

Lead-free assemblies compound the issues. Higher temperature requirements for lead-free reflow make it harder to avoid damage to sensitive components. To complicate matters, board assemblers may also encounter issues with hybrid boards that have a mix of leaded and lead-free components.

Reflow profiles that do not take all the above into account result in settings that produce unacceptable solder joints, failed components and overall low reliability. Enter thermal profiling, the process of mapping the thermal effects that a reflow process has on the solder, components and PCBs that make up an assembly. The tool that has driven advances in thermal profiling is the M.O.L.E.® thermal profiler, which in essence is the eye of the engineer inside the reflow oven. Initially, the capabilities of thermal profilers were limited to measuring minimum and maximum slopes and temperatures found in the oven and on the PCB. Engineers attached thermocouples to the bare PCB and beneath components to determine the delta T (ΔT) between the different locations. This allowed the engineers to minimize thermal exposure to the most heat-sensitive parts while ensuring a proper reflow of the higher mass components.

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