Heat Sink Solution
The customer, Photocircuits, Glen Cove, N.Y., is a producer of printed circuit boards (PCBs) and related assemblies. For this application, a DaimlerChrysler powertrain control module cover/heat sink assembly, the heat sink needed to be thermally conductive yet electrically isolated. The circuit board buildup 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 multilayer circuit boards. A typical solution involves 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 board and the aluminum heat sink, any contact between the board and the heat sink would result in scrap. Thus, the goal was to 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.
All of 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.
Buhrke engineers took up this set of concerns and devised a vibratory bowl feed and nesting channel in-die system (figure 1) for insertion of three ground posts (figure 2) into the heat sink. The precision and accuracy of placement, despite the high-speed run time, is key. 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 Buhrke-designed and engineered cams 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 Buhrke for this highly proprietary system.
The automation of this proprietary Buhrke system overcame the third challenge of eliminating labor from the entire assembly process. The three ground posts are precisely inserted in the heat sink at a rate 50 times faster than a conventional, labor-intensive and far less accurate secondary hard operation. Also, the automated solution 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 proximity sensors precisely monitor all aspects of this process.
A documented $1,000,000 savings over the life of the program has been achieved for the customers as a result of the in-die component insertion system. 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 sheet-metal brackets. To cut this cost, the 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. PH
Buhrke Industries, Arlington Heights, Ill., manufactures automated assembly systems for mechanical assemblies. For more information from Buhrke Industries, call (847) 981-7550 or visit www.buhrke.com.