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Implementation of a Temperature Ramp Rate Requirement and Impact on the Packaging Processes
Keywords: Temperatuare, Rate, Packaging
This paper addresses implementation of a heating and cooling temperature ramp rate requirement and its impact on packaging processes. A complex multi-chip module packaging design includes a printed wiring board, solder attachment of a AlSiC-9 back plate, solder attachment of a multi-room seal ring, soldered surface mount components, soldered power die directly to a heat sink, epoxy attached chip-and-wire die and a welded lid. Concerns arose about temperature excursions, because of complex material interactions that exist in the design. In addition, the design required the assembly process to be completed in an order that contributed to the need for a temperature ramp rate requirement. From a manufacturability standpoint some temperature spikes are unavoidable, so it came down to what short term temperature rate excursions would be acceptable to the product. The assembly challenge that resulted was a product requirement to limit the heating and cooling temperature ramp rates during packaging processes. Working together, the design agency and production agency came to an agreement on what the temperature ramp rate requirement should be and how to document the processes and the periodic determination of compliance. A thermal ramp rate was defined as a change in temperature over an interval of time, ramp rate = ∆T/∆t. A 1.5 °C/sec average heating and cooling ramp rate requirement over a 25 °C temperature delta was ultimately agreed upon. The 25 °C or less delta in temperature was included to allow for short term temperature spiking that might occur. The temperature spiking will occur during the placement of a cool part onto a hot surface (standalone hot plate for preheating or machine work surface) or the transfer from one hot surface to another surface set at a different temperature. Spiking will also occur when placing a hot part on a cool surface (cooling block after completion of heated assembly process). The requirement was implemented on all packaging processes that saw a heating and cooling cycle. Final assembly processes (after surface mount solder and all other soldering process), which include epoxy cure after die attach, wire bonding and die replacement rework processes, were impacted most significantly and are addressed in this paper. These processes were modified to meet the requirement. Solutions were implemented to minimize the impact to assembly flowtime and minimize the chance for process errors. In addition, the cost of implementation was considered with every attempt made to utilize existing equipment and to use simple tooling to minimize the cost. The design agency and production agency worked collaboratively, utilizing thermal profiling, simulation modeling, tooling design and material evaluations. Extensive thermal profiling was completed initially to document the as-is process and to better understand the challenges of the requirement. More thermal profiling was completed to evaluate tooling materials and to document the new temperature settings. Thermal profiling was done by attaching thermocouples to critical locations, on an assembly, then using a data acquisition logger to collect the data. The thermal data was evaluated and compared with computer simulation data. In addition, simulation was used to evaluate different tooling deigns, including different material types, mesh sizes and thickness. The tooling would be used in conjunction with hot or cool surfaces to slow down and control the heat and cooling ramp rates. The final assembly processes were impacted to varying degree, with heating and cooling rates being addressed separately. Cool down from peak process temperatures was addressed simply by using off the shelf cooling racks normally used for baking. Prior method used a solid aluminum cooling block. A cooling rack minimizes the surface contact with the hot assembly. In addition, an exception was taken to the normal ESD requirement of ionization fans blowing on assemblies during heating and cooling. During initial temperature profiling it was determined the ionization air stream was an uncontrolled variable that had a significant impact on the heating and cooling ramp rate. Temperature rate programmable ovens were used to meet the requirement for epoxy cure and other bake out processes. A temperature rate programmable hot plate was used to heat assemblies during epoxy component rework. The hot plate being large enough for several assemblies. Preheat for the wirebonding process proved to be most challenging. Using a programmable hot plate that would heat one part at time then have the hot plate cool back to room temperature, to be ready for the next part, added too much additional time to the process. Ultimately an intermediary material was developed that would be placed between hot plate and assembly. The hot plate would be set at a static temperature. A tool make of the intermediary material would be sandwiched between hot plate and assembly. In addition, the hot plate would get the part to 125 °C before moving it to the 150 °C wire bond stage. The 25 °C delta between the hot plate and work stage would have a minimal temperature spike and not violate the requirement. Temperature ramp rates processes were documented and qualified based on the assembly machines, ovens, hot plates and tooling used as well as set point temperatures and dwell times. Initial compliance was proved through thermal coupling of an assembly and thermal profiling. Deviation from the process would require approval from the design agency.
Randy Hamm, Engineer Principal Electrical
Kansas City National Security Campus, Honeywll
Kansa City, MO

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