Authored By:
Rachel-Anne A. Stupp
Honeywell Federal Manufacturing and Technology
MO, USA
Summary
Working in a Class 3 manufacturing facility requires high reliability processes, process controls, and the understanding of the control effects. Each board is unique in design and can result in unique component placement making rework difficult, forcing production agencies to get creative in reflow processes.
Understanding the rework equipment and proper use can greatly impact the success of a solder reflow process while maintaining heat sensitive requirements and part safety. Board thickness, component location in relation to heat source, and the presence of an ionizing fan are a few of the various control factors that have been identified to solidify a repeatable rework process.
The conducted experiment comprised of 2 bare boards, a 13 layer 3”x 3.25” board and a 20 layer 2.5”x 6.5” board, at 2 different heights from the hot plate. The bare boards help determine a general heat range without component heat blockage or absorption. Measurements of the board temperatures were taken in various increments in multiple locations on both sides of the board. The temperature of the hot plate was kept constant but was interesting to see how it fluctuated based on the board thickness, board height from the hot plate, and the distance from the ionizing fan. These results can be used to better maneuver the reflow equipment to protect components and boards against thermal shock and damage.
Identifying the controllable vs. un-controllable factors can help reduce inconsistency in reflow results. Some un-controllable factors can also be manipulated in a way to benefit the solder reflow process. This process is not solely contained to rework but rather to precise solder reflow in a general sense. As rework and solder reflow become ever present as a part of normal processing, it must be accepted that hand installation is still an important method to improve and design into the manufacturability of a board.
Conclusions
The main control determined from this study was that the placement of the ionizing fan greatly affected the temperature read outs, a lower temperature near the fan but also sort of pushing the heat away from the fan. An assumption that was found true was that the heat from the hot air penetrates better through the thinner boards. The low board stand height was chosen to be the best option for its consistency of heat throughout the trials. The fan did not affect the convection the bottom side of the board and hot plate were creating when using the low stand height. The height chosen for the hot air nozzle was too far away from the board for normal production reworks, it will need to be explored more for an accurate and precise distance. Upon completion of all trials, the decision was made that the method in which the data was captured should be improved from single point to continual temperature monitoring.
It was also noticed when using the 9th thermocouple input on the system that there was a built-in temperature control feature. When first using the 9th input a temperature max of 200C was set and therefore would decrease the heat on the hot plate when the thermocouple read higher than 200C. For this study, it was a hinderance and therefore not discussed, but has the potential to benefit future production and processes. Another round of trials will need to take place to fully understand how the heat dissipates across the board when the board is fully populated, applying heat shields to heat sensitive components, and how the placement of the ionizing fan could help protect heat sensitive components where the design cannot be changed.
Initially Published in the SMTA Proceedings
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