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www.us-tech.com
June, 2013
Build HDI Structures With Thin Films and Low-Temperature Sintering Paste
By Catherine Shearer, James Haley, and Chris Hunrath, Ormet Circuits, Inc., San Diego, CA
tion capabilities. As packaged and chip electronic devices grow in complexity, so too do the circuit boards on which they are mounted. PCB fabrication must improve to keep pace, and PCB designers and fabricators do this through the use of multilayer cir- cuits, high-density-interconnect (HDI) structures, and interconnecting viaholes in almost any location on any circuit layer. Materials are being developed for manufacturing these HDI PCBs, but they must also meet today’s electrical and thermal require- ments, while providing high reliability. By using these new materials, it should be possible to meet the performance needs of advanced HDI PCBs with- out “re-inventing” the PCB shop. Electronic components and devices continue
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to grow in complexity, with more functions in smaller areas, forcing PCB designers to expand from simple single-sided PCBs to more complex multilayer PCBs which require advanced laminat- ing and plating processes. But building HDI PCBs is not simply about achieving high layer counts, since adding circuit layers can have a dimension- ing return: the required via holes and througholes to connect layers can occupy valuable circuit real estate. Buried via holes and “subs” provide an intermediate solution to high-density require- ments, but using these techniques also increases PCB manufacturing complexity and fabrication time (and cost). As the trend for high-density circuits contin-
ues, multilayer circuits become more common- place; circuit build-up and sequential lamination have grown in popularity as improved laser drilling and plating technology have simplified the creation of blind microviaholes between circuit lay-
rowing demands for increased circuit func- tionality continue to drive the need for improved printed-circuit-board (PCB) fabrica-
ers, and the creation of multilayer circuits. With sequential circuit build-up approaches, viaholes can be placed anywhere on any layer to connect different parts of a circuit. Still, each circuit layer requires the process steps of an individual PCB, boosting the total laminating and plating require- ments. Unfortunately, the gap is growing between cost-effective PCB manufacturing and HDI PCB
before circuit lamination? This would greatly reduce PCB production steps for plating and lami- nation, but would require insertion of some form of conductor into the via holes in a PCB’s dielectric material, to make electrical connections between the individual circuit layers. This can be done in several ways, using C-stage (cured) dielectric materials or B-stage (uncured) dielectric materi- als. Ideally, the conductor would be a paste or in liquid form so that it can be inserted into the via- holes after they are drilled but before lamination. Before the conductor is applied, the surface of the dielectric material must be free of contamination of any conductive residue that can lead to electri- cal leakage or other problems. By creating via holes prior to lamination,
multilayer PCB manufacturing can be greatly sim- plified while still enabling placement of via holes anywhere on the PCB on any layer. The formation of conductive via holes prior to lamination also lends itself to traditional PCB manufacturing processes for assembling and laminating circuits, for efficient and cost-effective use of a PCB shop. Transient liquid phase sintering (TLPS) is a
This is a laser-drilled, paste-filled via hole prior to lamination.
requirements. New materials and technologies are needed to help reduce this gap for current and future multilayer circuit requirements. Obviously, via holes are essential to multilay-
er circuits and HDI PCBs. What if they could be formed more simply, without need of plating, or
process by which a liquid metal, such as tin, will interdiffuse with a nonmolten metal, such as cop- per, to form a solid bond or joint between the two metals at relatively low temperatures. This form of interdiffusion yields a metallurgical bond between two metals that is stronger than a mere layer-to- layer interface. Interdiffusion between metals has been used
in electronic assembly for some time to form envi- ronmentally sound and strong electrical intercon- nections. For example, solder joints rely on inter- diffusion between copper and tin.
Continued on page 58
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www.apem.com
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