April, 2011
www.us-tech.com
Page 51 Vacuum Bonding for High-Yield Production By Franziska Weichhaus, Director of Marketing, Scheugenpflug AG
igh-power operation of assem- blies and closely packed com- ponents require effective heat transfer to a heat sink that is in ther- mal contact with the PCB. Inefficient heat transfer causes assemblies to overheat, which may damage the electrical components or result in failures in operation. The heat dissi- pated by the components is trans- ferred from the assembly to the heat sink’s cooling surface, which is usual- ly made of metal (aluminum), by thermally conductive adhesives. These materials fill the voids present in the interface between the two objects caused by surface rough- ness or similar discontinuities. Adhesives are ideally suited for this purpose, because of their strainability and moldability. However, their high viscosity often poses greater chal- lenges in the joining of two objects, especially when pressure is applied during production. A novel processing technology offers a rapid and highly economic solution for this problem.
H
A new vacuum bonding process developed by Scheugenpflug AG for Dow Corning is more advanced and offers higher economic efficiency than predecessors.
Thermally conductive adhesives are highly filled one- or two-compo- nent materials. They can be applied to the heat sinks by stamping or by dispensing, depending on the re- quired coat thickness. Usually, the more filler materials they contain, the higher is their thermal conduc- tivity. At present, the thermally con- ductive adhesives in use display a filling material content of up to 70 percent. To fulfill the engineering requirements for heat transfer, the content of fillers has frequently been increased. The limit is not the capac- ity for fillers, but the capability of equipment to handle such materials. Due to their paste-like consistency, highly filled thermally conductive adhesives (density in excess of 3g/cm3) cannot be applied to the sur- faces of heat sinks by stamping.
Highly Viscous Materials
Alternatively, such highly vis- cous materials can be applied with available dispensing technology. The slightly longer processing times are easily compensated by dispensing the beads of adhesive at a coarser pitch. The true challenge in both tech- niques, however, turns out to be the pressure necessary to effectively affix the heat sink to the circuit board. Spreading the thermally conductive adhesive evenly between the two parts without trapping air, which requires pressing them together with considerable force, has been an unre- solved problem so far. The reason is mainly the high viscosity of thermally conductive adhesives. Although press- ing the parts together with the aid of pressure pins achieves sufficient pres- sure distribution across the die, this method is not without risks. It often results in the ceramic dies cracking due to excess pressure being applied. All the more so when pressure distri- bution does not occur uniformly
across thin and wide-stretched ceram- ic substrates (PCBs).
Dow Corning, the renowned manufacturer of high-effi- ciency next generation ther- mally conductive adhesives, was looking for a technically more advanced solution that also offers higher economic efficiency. The new vacuum bonding process developed by Scheugenpflug AG has been designed to solve the afore- mentioned problems and ful- fils all the requirements, including process safety. The results are perfect: After
applying the bead of adhesive to the heat sink surface and placing the PCB on the heat sink, the still unfin-
ished assembly is introduced into a small vacuum chamber. The small size of the vacuum box (less than 0.002m3) allows evacuation and sub- sequent ventilation within fractions of a second.
Heat sink (gray), thermally conductive adhesive (yellow), AND ceramic PCB (white).
During evacuation, air is removed completely, even the air trapped between the beads of adhe- sive, as well as all the air between heat sink and board. When air is introduced afterwards, the instantly rising air pressure bonds the heat sink tightly and evenly to the surface of the PCB. The distance between the two surfaces is determined by grains
Continued on page 58
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