Note: This is the second of a three-part series covering metal injection molding (1) trends, (2) material/process advances and (3) applications.

Within the field of advanced metal injection molding (MIM) technology, aluminum MIM thermal management for electronics is a major focus for materials and process development.

Even the best-designed most dependable electronic device can fail if it overheats. Electronic component suppliers estimate that device failure rate doubles for every 10 degrees C rise in junction temperature. Thus, if the waste heat generated inside an electronic device package is not removed, the reliability of the device is severely compromised.

According to Moore's Law, the amount of information storable in one square inch of silicon has roughly doubled every two years. Moore's Law also applies to thermal management.

As chip technology becomes increasingly smaller and more powerful, the amount of heat generated per square inch increases accordingly. Current processors generate power densities in the order of 10-40 Watts per square centimeter (W/cm2) with this expected to increase to 20-60 W/cm2 in the near future.

State-of-the-art integrated circuits for microprocessors operated at high frequencies are routinely characterized by power densities 50 W/cm2. Large density will lead to highly localized heating or interconnect "hot spots."

Consumer demand is driving products and technologies toward further miniaturization and more portable power consumption. Simultaneously, users want more capabilities in these smaller, more powerful platforms.

Assembly and packaging technology changes in response to these demands is making the thermal management situation more complex, driving new approaches to cooling. When simple thermal solutions cannot remove enough heat to maintain component reliability, the system designer must look to more sophisticated measures, such as heat sinks.

Passive heat sinks use a mass of thermally conductive material to move heat away from the device into the air stream, where it can be carried away. Heat sink designs typically include fins or other protrusions to increase the surface area, thus increasing its ability to remove heat from the device. Segmenting the fins can further increase the surface area to get more heat removal in the same envelope.

Passive heat sinks optimize both cost and long-term reliability. Increasingly, thermal performance challenges are mitigated by using novel tungsten-copper, boron nitride crystals and copper–multiwalled carbon nanotube materials in heat sinks.

Advanced Materials Technologies
Molded aluminum tapered fins heat sink.

For example, Singapore-based Advanced Materials Technologies (AMT) has commercialized an aluminum metal injection molding (MIM) process that will "disrupt" current ideas about thermal management, power consumption and the limits of miniaturization in electronic and automotive product designs.

AMT has molded aluminum heat sinks with tapered fins, a design that aerodynamically maximizes surface area to improve thermal management. AMT's technology allows it to mold heat sinks that can direct airflow in custom-designed directions.

Excellent electromagnetic and radio frequency interference (EMI/RFI) shielding, thermal conductivity and walls as thin as 1 millimeter are possible with colorable and platable aluminum MIM electronic enclosures. AMT also has the processing savvy to mold other thermal management materials, such as copper and tungsten-copper.

AMT's aluminum MIM process delivers a disruptive one-two punch. On one hand, it can provide significantly improved thermal conductivity compared to aluminum parts manufactured by conventional means such as casting, extrusion, machining and the like. On the other hand, the thermal management properties of aluminum MIM parts are isotropic and therefore uniform in all directions.

Continuing, new MIM heat sink materials for microprocessors are being rapidly developed. Metal injection molding (MIM) used for heat sink manufacturing in place of metal extrusion or machining creates a dense structure with broader geometry options.

The growing power requirements and shrinking size of high-performance microprocessors presents ever-increasing thermal loads and greater heat dissipation challenges to microelectronic package designers. New heat sink designs are needed to cope with these increased demands, and injection molding is potentially a key element of the solution.

A primary consideration is to maximize the heat sink's thermal conductivity while matching the thermal expansion coefficient of semiconductor materials like silicon (4 parts per million, per degree Centigrade; ppm/ºC). Affordable metals with high thermal conductivity such as aluminum and copper do not exactly match silicon's thermal expansion.

The resulting stress between the silicon and heat sink created during every temperature cycle eventually causes the assembly to delaminate and the circuit to overheat. A major objective is to develop a heat sink material with high thermal conductivity that also has a thermal expansion coefficient equivalent to typical semiconductor materials (4-7 ppm/ºC).

New tailored composite materials, such as tungsten-copper, molybdenum-copper and silicon carbide-aluminum have been developed to meet this challenge, combining excellent thermal conductivity with thermal expansion coefficients that are appropriate for many electronic packaging applications.

Pennsylvania State University
Molded aluminum tapered fins heat sink.

Tungsten-copper (W-Cu) is one material of particular interest. It has uniquely high thermal conductivity in the 4-7 ppm/ºC thermal expansion coefficient range relative to other electronic alloys, matched only by diamond or toxic beryllia.

W-Cu with 15-20 percent weight copper is formed by mixing powders and can be metal injection molded into complex geometries and sintered to meet heat sink design requirements. Since copper is poor at promoting sintering densification even well above its melting point of 1083 degrees C, fine powders are needed to achieve a high sintered density for W-Cu. Composite powders such as Osram Sylvania's Tungstar W-Cu powders, produced from co-reduced copper and tungsten oxides can meet these requirements and also significantly reduce sintering temperatures.

The ongoing evolution of heat sink designs provides an interesting opportunity for metal injection molders to provide advanced solutions for the thermal management problems confronting the electronics industry. The Center for Innovative Sintered Products (CISP) located at Pennsylvania State University is active in commercializing thermal management materials formed by powder injection molding and sintering.

Several commercial products have been developed in conjunction with AMTellect Inc. based on molybdenum copper, aluminum nitride, invar silver and now tungsten copper.

Finally, with regard to automated micro-MIM production, there are surely differences between metal injection molding and molding plastic, but in one important sense they are exactly the same. If you can automate the process for an extended run, your chances of making a profit increase dramatically.

A group of organizations proved it could be done, even when the MIM part is a micro-gear wheel with a finished diameter of 1,200 microns and a thickness of only 200 microns.

The all-German team consisted of The Institute for Machine Tools & Production Science of the University of Karlsruhe; Kugele, a mold maker specializing in tools for micro parts (less than 120 grams); and machinery equipment supplier Arburg. Each of them faced significant challenges in making this project work, each of which related to the need for ultraprecision.

The production machine was a 50-ton Arburg Allrounder 320 C with a 15-mm MIM-geometry plasticating screw featuring reduced compression and modified zone splitting. For this gear, the shot volume is 1.1 cubic centimeters and the injection flow rate is 10 cubic centimeters per second. The switchover pressure is 900 bar (13,053 psi); holding pressure time is 0.5 second; and the pressure decreases from 900 bar to 25 bar (363 psi) in 0.5 seconds.

Arburg found that the 15-mm screw is very sensitive to the smallest process changes, which permitted filling analyses to be done with switchover point parameter steps of .005 cu cm to determine optimum cavity filling.