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Online Case Study: In-Die Components Allow Heat Sinks Thermal Conductivity, Electrical Isolation
December 31, 1969 |Estimated reading time: 4 minutes
By Ken Wesseln, Harold Breen, Paul Kozlowski and Keith Krutz
Through a proprietary system of in-die component insertion, a contract metal stamper* has helped one customer achieve thermal conductive and electrically isolated conditions on a 5052 aluminum heat sink assembly for an automotive powertrain control module.
The customer, Photocircuits (Glen Cove, N.Y.) is the largest producer of printed circuit boards (PCBs) and related assemblies in the world. For this particular application, a DaimlerChrysler powertrain control module cover/heat sink assembly, 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 DCX electronics plant.
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 the desired results were not being achieved. With only a 0.004" layer between the PCB and the aluminum heat sink, any 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 secondary operation, plus there were anticipated inconsistencies.
Buhrke, itself a Tier One supplier to DCX, suggested the inclusion of a ground post soldering concept. 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. Plus, the grounding location was near the outside edge, while the Buhrke solution 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 for Photocircuits.
Engineers from the contract metal stamper took up this set of concerns and devised a vibratory bowl feed and nesting channel in-die system for insertion of three ground posts (locations shown in photo) into the heat sink. The precision and accuracy of placement, despite the high-speed run time, is key here. 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 back side of the heat sink for proper soldering.
A system of cams designed and engineered by the contract metal stamper push the three ground posts into their proper position in-die. The entire tool production and all the ancillary equipment assembly were done in-house by the contract metal stamper for this highly proprietary system.
The automation of this proprietary 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, in-die. Piezoelectric force measurement and conventional prox sensors precisely monitor all aspects of this process.
The three ground posts are being precisely inserted in the heat sink, it should be noted, at a rate 50 times faster than a conventional, labor-intensive and far less accurate secondary hard operation.
Working closely with Photocircuits and DCX engineers, the contract metal stamper did all the production tooling, die design, assembly, set-up, prototyping and full production verification in-house for this project.
A documented $1 million savings over the life of the program has been achieved for the customers as a result of this unique in-die component insertion system.
DaimlerChrysler engineering confirmed the performance of this solution. 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, introduction of RF ground posts was suggested to bond the PCB directly to the heat sink, with the solder liquifying around the ground post (thus the need for tin plating). An overall total seal was achieved, once the module connector was in place, without RF ground to the chassis, which had created related problems such as tachometer flutter. The mounting screws into the chassis would now contact the aluminum heat sink, thus achieving the shortest possible RF ground path. This electromechanical grounding system was found far more efficient and overall much more reliable than any mechanical system, according to DCX engineering. In the final analysis, after many work-ups and prototypes, the system designed by the contract metal stamper "...was the best and the most effective solution to our challenge," stated an engineering supervisor at DCX.
The contract metal stamper reports there were no modifications required to the 150-ton Minster presses on which this job currently runs.
*Buhrke Industries (Arlington Heights, Ill.)
For more information, please contact: Buhrke Industries Inc., 511 West Algonquin Road, Arlington Heights, IL 60005-4499. Phone: 847-981-7550. Fax: 847-981-0772. Web: www.buhrke.com. Email: mailto:info@buhrke.com. Attention: Ken Wesseln.