Sinking Heat, EMI/RFI

In-die components provide thermal conductivity, electrical isolation

Through a proprietary system of in-die component insertion, contract metal stamper Buhrke Industries, Arlington Heights, IL, has helped Photocircuits, Glen Cove, NY, a producer of printed circuit boards and related assemblies, achieve thermal conductive and electrically isolated conditions on a 5052 aluminum heat sink assembly for a DaimlerChrysler powertrain control module cover. For this application, the heat sink needed to be thermally conductive yet electrically isolated. The circuit board build-up was to be bonded to the heat sink and post-cured in a vacuum laminating press. Finally, the entire module assembly was to be done at the DaimlerChrysler electronics plant (DCX) in Huntsville, AL.

The first challenge was to address the considerable heat present in the small package of multi-layer circuit boards. In many automotive and other applications worldwide, a typical system involved an epoxy adhesive bonding of the circuit board to the stamped aluminum heat sink and powertrain control module cover. Various adhesive systems were attempted but none achieved the desired results. With only a 0.01 mm layer between the PCB and the aluminum heat sink, any unintended contact between the board and the heat sink would result in scrap. Thus, the goal was pull the maximum amount of heat off the PCB without making contact. An adhesive would function, but the PCB still needed to be grounded to the module.

Soldering or hard-brazing as secondary operations were explored, but cost projections were running in the range of $250,000 annually for this process, and inconsistencies were anticipated. Buhrke suggested ground post soldering. DCX engineers suggested a copper plate, but the costs were again substantial. After considerable trial and error, Buhrke engineers devised the inclusion of several tin-plated brass ground posts inserted into the heat sink. The tin plating would overcome the brass/aluminum incompatibility in soldering, yet provide sufficient electrical isolation on the heat sink.

All the other grounding systems under review were mechanical, including screws through the aluminum and a drawn housing to interface with the solder pads, as well as cold bonding of the brass and aluminum. These were deemed too unreliable. Additionally, the grounding location was near the outside edge, while the Buhrke design brought the ground posts closer to the connectors for better EMI/RFI reduction.

Positioning was the second challenge. Ground post height, diameter, parallelism and location were all major concerns. The engineers devised a vibratory bowl feed and nesting channel in-die system for insertion of three ground posts into the heat sink. The precision and accuracy of placement, despite the high-speed run time, was of key importance. The ground post, which measures only 0.79 mm in height, must maintain a ±0.13 mm clearance on the top of the heat sink and a zero clearance on the backside of the heat sink for proper soldering. In the new design, a system of cams pushes the three ground posts into their proper position in-die. The entire tool production and all the ancillary equipment assembly were done in-house.

The automation of this system overcame the third challenge of eliminating labor from the entire assembly process. This automation scenario includes not only the assembly step reduction, but also the inspection process, as nine electronic sensors in each press check the force and the positioning height of the ground posts, during and after insertion into the heat sink, all done in-die. Piezoelectric force measurement and conventional proximity sensors precisely monitor all aspects of this process. It should be noted that the three ground posts are precisely inserted in the heat sink at a rate fifty times faster than a conventional, labor-intensive and far less accurate secondary hard operation.

There had been substantial cost issues related to heat sinking the components on this module. The RF ground from the PCB was passing into the housing through an RF shield soldered on the board, then to the vehicle through screws into the chassis sheetmetal brackets. To cut this cost, RF ground posts were suggested to bond the PCB directly to the heat sink, with the solder liquifying around the ground post (hence the need for tin plating). Once the module connector was in place, an overall total seal was achieved without RF grounding to the chassis, which in tests had created related problems, such as tachometer flutter. The mounting screws into the chassis now contact the aluminum heat sink, thus achieving the shortest possible RF ground path. After many work-ups and prototypes, the Buhrke-designed electromechanical grounding system is more efficient and overall more reliable than mechanical systems, and “…was the best and the most effective solution to our challenge,” stated an engineering supervisor at DCX. Buhrke reports there were no modifications required to the 150-ton Minster presses on which this job currently runs. DCX confirms that this in-die component insertion system has saved $1 million over the life of the program.

—SG

For more information:
Buhrke Industries
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Photocircuits
www.rsleads.com/307df-162

Daimler-Chrysler Electronics Plant
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