|Heat sink (gray), thermally conductive adhesive (yellow), AND ceramic PCB (white). |
High-power operation of assemblies and closely packed components require effective heat transfer to a heat sink that is in thermal 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 dissipated by the components is transferred from the assembly to the heat sink's cooling surface, which is usually made of metal (aluminum), by thermally conductive adhesives. These materials fill the voids present in the interface between the two objects caused by surface roughness or similar discontinuities. Adhesives are ideally suited for this purpose, because of their strainability and moldability. However, their high viscosity often poses greater challenges 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.
Thermally conductive adhesives are highly filled one- or two-component materials. They can be applied to the heat sinks by stamping or by dispensing, depending on the required coat thickness. Usually, the more filler materials they contain, the higher is their thermal conductivity. At present, the thermally conductive 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 capacity 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 surfaces of heat sinks by stamping.
Highly Viscous Materials
Alternatively, such highly viscous 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 techniques, 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 unresolved problem so far. The reason is mainly the high viscosity of thermally conductive adhesives. Although pressing the parts together with the aid of pressure pins achieves sufficient pressure 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 distribution does not occur uniformly across thin and wide-stretched ceramic substrates (PCBs).
Dow Corning, the renowned manufacturer of high-efficiency next generation thermally 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 aforementioned problems and fulfils 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 unfinished assembly is introduced into a small vacuum chamber. The small size of the vacuum box (less than 0.002m3) allows evacuation and subsequent ventilation within fractions of a second.
During evacuation, air is removed completely, even the air trapped between the beads of adhesive, 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 that are added to the adhesive. These solid particles are homogeneously distributed throughout the thermally conductive adhesive and determine the width of the gap across the interface between the two objects. The size of the grains defines the distance.
Piston Metering Needed
However, not all dispensing systems are capable of carrying out this process. The ideal systems for the task are piston metering heads that are designed for handling large grains contained in the dispensing mass and suited for abrasive and highly filled adhesives even in long-term operation.
This processing technique precludes the need for costly stamping presses while reducing the danger of cracking the dies due to unevenly applied pressure. Another advantage: the circuit board design is less restricted, because no surfaces are needed to place the pressure pins. At the same time, the process eliminates the risk of damaging the circuit board or the mounted electrical components and conductive pathways. Furthermore, the dispensing technology combined with subsequent vacuum bonding allows cycle times comparable to those achieved with pressure stamping, because of the small size of the vacuum box. This lets the air be evacuated and reintroduced within the shortest possible time.
Using extremely filled thermally conductive adhesives followed by air-bubble-free vacuum bonding with a reserved interface distance, effective heat management can be achieved at production quality levels previously unknown.
Contact: Scheugenpflug, Inc., 2125 Barrett Park Dr. NW, #104, Kennesaw, GA 30144-3613 770-218-0835 Web: http://www.scheugenpflug-usa.com