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Build HDI Structures With Thin Films and Low-Temperature Sintering Paste
This is a laser-drilled, paste-filled via hole prior to lamination.
By Catherine Shearer, James Haley, and Chris Hunrath, Ormet Circuits, Inc., San Diego, CA
Growing demands for increased circuit functionality continue to drive the need for improved printed-circuit-board (PCB) fabrication 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 circuits, 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 requirements, while providing high reliability. By using these new materials, it should be possible to meet the performance needs of advanced HDI PCBs without “re-inventing” the PCB shop.
Electronic components and devices continue 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 laminating and plating processes. But building HDI PCBs is not simply about achieving high layer counts, since adding circuit layers can have a dimensioning 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 requirements, but using these techniques also increases PCB manufacturing complexity and fabrication time (and cost).
As the trend for high-density circuits continues, multilayer circuits become more commonplace; 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 layers, 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 requirements. Unfortunately, the gap is growing between cost-effective PCB manufacturing and HDI PCB 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 multilayer circuits and HDI PCBs. What if they could be formed more simply, without need of plating, or before circuit lamination? This would greatly reduce PCB production steps for plating and lamination, 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 materials. Ideally, the conductor would be a paste or in liquid form so that it can be inserted into the viaholes 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 electrical leakage or other problems.
Circuit materials filled with glass fibers can suffer melting of the end of the fibers, resulting in “slag” droplets.
By creating via holes prior to lamination, multilayer PCB manufacturing can be greatly simplified 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 process by which a liquid metal, such as tin, will interdiffuse with a nonmolten metal, such as copper, 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 environmentally sound and strong electrical interconnections. For example, solder joints rely on interdiffusion between copper and tin.
By formulating a conductive paste with TLPS characteristics, sintering of the metals comprising the paste will occur at normal PCB laminating temperatures (as low as +180°C). Using tin in such a TLPS conductive paste also provides the benefit of forming metallurgical bonds not just through the paste interconnection, but through the copper foil circuitry, in the manner of solder. However, unlike solder, the TLPS conductive paste will not wet beyond the footprints of its target via holes and will not re-melt during subsequent processing. And like plated via holes, interconnections formed with TLPS paste provide a continuous metallurgically bonded electrical pathway.
Within a TLPS conductive paste viahole, copper and alloy particles “micro-weld” together to form a mechanically and electrically sound connection. The re-melt temperatures of the various phases of these materials are well above reflow temperatures and, for all intents and purposes, nonreversible in the PCB.
Bonding films not reinforced by glass fabric provide many advantages for HDI structures both in fabrication and signal integrity. While laser drilling is normally not performed on B-stage materials, it is required for the prelamination viahole formation sequence. In glass fabric circuit materials, the ends of the glass fibers tend to melt from the heat of the laser, forming “slag” droplets at the end of the fiber bundles. But polymer bonding films, even in B-stage form, tend to provide more consistent results with laser drilling. This is due to the uniform composition: polymer without the glass. In any case, the laser heat source must be matched to the bonding film, since optically transparent films do not respond well to UV laser energy.
A variety of bonding films are available for use with flexible PCB materials. Because the films are commonly machined prior to lamination, they are formulated with reduced-flow characteristics. Such flow behavior is also good for use in a TLPS paste. Unfortunately, flexible films contain one or more plasticizing agents which provide flexibility but are also integral to flow control. These plasticizers readily soften with applied heat and exhibit large amounts of thermal expansion, which are traits not well suited for HDI structures. Of course, HDI PCBs are free from the dynamic bending requirement, and perhaps it might make more sense to fabricate them on PCB materials and bonding films that are not nominally “flexible.”
This less-flexible circuit material is formed by replacing the “rubberized” matrix of a flexible circuit with a polymer matrix that behaves more like a glass weave, but without the negative attributes of a glass weave. This proprietary high-temperature polymer is crosslinked to achieve the benefits of woven glass without its differential lasing characteristics, poor dielectric performance, and resin wetting problems. The polymer matrix is combined with a high-temperature B-stage resin with good bondability and long shelf life. The combination is a material system suitable for use with TLPS paste and interconnections.
By formulating a dielectric film suitable for both the paste interconnect process and HDI PCB form factors, implementation of multilayer circuits with TLPS z-axis interconnections becomes straightforward. The different material components are designed to work together, with minimal impact to standard PCB manufacturing processes, in support of thin, high-density PCBs with excellent electrical performance. Through proper flow control of the resin, well-defined viahole shapes can be maintained for reliable electrical and mechanical bonds to a circuit’s copper pads. These thin-film circuits achieve dense, continuous interconnections that are consistent from pad to pad and via hole to via hole.
The film system is not dependent on glass fabric, thermal aging, or other factors for flow control, so the resin effect on a TLPS viahole is predictable and consistent over a long shelf life. The film is a stand-alone dielectric material without the electrical and thickness constraints imposed by glass, but with superior thermal properties similar to a lead-free-compatible, glass-reinforced system. TLPS interconnects can be short and placed anywhere on a PCB, for an extremely thin multilayer structure. The material system enables an HDI process that can be readily implemented by a PCB shop while providing the same reliability as copper-plated-and-filled micro via holes.
Combining a TLPS paste with a film that is engineered without flexible-material constraints can produce PCBs with thermally reliable z-axis interconnects. The TLPS paste offers a reliable metallurgical bond to the inner copper layers while the film allows proper sintering and controlled z-axis expansion. Together, these materials offer producers of multilayer PCBs ease of implementation of high reliability sintered-paste interconnects and more construction options for HDI structures.
Contact: Ormet Circuits, Inc. — Integral Technology, 6555 Nancy Ridge Dr., San Diego, CA 92121
858-831-0011 fax: 858-455-7108 E-mail: chunrath@Integral-HDI.com Web:
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