Miniaturization and better performance are the driving forces in the design of densely populated printed circuit boards.
"Real estate on pcbs keeps shrinking, so you have to think small to get big results," said Chris Marx, director of electronics business development for Textron Fastening Systems. "Miniaturization is a big word that simply means design it as small as you can, and when you are finished, go back and make it smaller."
For the past year and a half, Marx and a team of Textron Fastening Systems engineers have been thinking small in collaboration with Volterra, a fabless semiconductor company in Fremont, CA. Volterra designs custom ICs for low-voltage, high-current, point-of-load power supplies.
Volterra reference designs are the industry''s smallest and lowest profile power supplies for microprocessors, memory, and other power-hungry ASICs in computer and datacom spaces. Because of their performance and density, these designs have been embraced in high-volume production by major U.S. computer and datacom OEMs.
"Our reference designs must meet all electrical, mechanical, and environmental specifications, including proper cooling during operation," said Ognjen Djekic, design manager for power modules for Volterra.
"Some module designs require an aluminum heat sink interfaced to the ICs and secured to the pcb," Djekic added. "The heat sink attachment process must be reliable, repeatable, compact, and suitable for high-volume production."
Focus On Miniature
Heat sink attachment was a major obstacle in a recent Volterra reference module for IU server applications. To solve this problem, Volterra asked Textron Fastening Systems to supply all mechanical needs for the power supply module, including fasteners, a heat sink, and an assembly process to ensure package quality and consistency in production.
"We delivered the total solution, from earliest CAD drawings to assembly processes to development of the most efficient service procedures," Marx said. "The focus was on miniaturization, without any compromises in performance. We had to provide a secure joint that would retain the heat sink in a fixed position."
Everyone involved agreed that riveting the heat sink to the pcb surface would be the most advantageous approach for the attachment process. "Rivets are easily and quickly applied, they have no torque control requirements, and can be removed and replaced for service with no damage to the pcb," Djekic said.
"Screws require tapping the heat sink, meaning increased costs for secondary operations, and they are difficult to automate and require precise torque control," he added. "Soldering the heat sink to the pcb wasn''t acceptable, since this is a manual process that makes rework difficult."
The Textron Fastening Systems rivets had to have consistent joint performance, a small footprint to accommodate space constraints on the pcb, accurate fastener applications, and consistent push out load in rework and service procedures, Djekic said. They also had to be adaptable to the varying dimensions and multiple hole alignments of all pcb platforms, allowing one rivet to fit many designs.
The result was an aluminum rivet with a 1.6mm diameter (1/16th in.), the smallest ever designed for a pcb. Based on the Textron Fastening Systems Avdel® brand Speed Fastening line, either two or four of these miniature fasteners are applied to the heat sinks, depending on platform configuration.
"The minute size of the fastener head and shank make it ideal for placement in precision assembly on a tightly populated pcb," Marx said. "The bonded joint provides high resistance to shear and loosening caused by system vibration and thermal cycling. In repair procedures, the rivet can be pushed out from the tail side while preserving pcb integrity."
60 Rivets Per Minute
The completed power supply module consisted of a pcb, mounted chipsets and components, and the aluminum heat sink. Upon completion of the project, Volterra provided assembly subcontractors and two power supply manufacturers with a complete reference design. Their assignment was to qualify the design, and develop and streamline high-volume manufacturing processes.
For the assembly process, Textron Fastening Systems redesigned an Avdel® 7353 Speed Fastening system that delivers up to 60 rivets per minute to one side of the pcb surface through preplaced holes. Designers added an air cursor for feeding the miniature rivets and a pulse valve to ensure correct seating before broaching. The hydra-pneumatic tool is mounted to a workbench, providing a flat surface and perpendicular orientation to the pcb and heat sink assembly.
A speed fastening mandrel and rivet are inserted through the clearance hole in the pcb and then into a specially designed mating hole in the heat sink. The rivet shank expands 20% as it descends into the heat sink and pcb surface, providing an interference fit of approximately .15mm (006 in.). The pcb clearance hole is preserved, and the material clenches together into a joint with structural soundness and high resistance to loosening.
At the end of each cycle, the next fastener is indexed to the nosepiece, ready to repeat the assembly process. When the rivet supply is depleted, the mandrel is ejected and a fully loaded replacement magazine is quickly inserted.
"Textron Fastening Systems has a long history of partnership with many of the world''s leading electronics companies," Marx said. "During this time, Speed Fastening application success rates have exceeded 99.5%."