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June, 2013 Continued from page 56
Build HDI Structures With Low-Temperature Sintering Paste and Thin Films By formulating a conductive
paste with TLPS characteristics, sin- tering of the metals comprising the paste will occur at normal PCB lami- nating 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 cop- per 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, inter- connections formed with TLPS paste provide a contin- uous metallurgically bonded electrical pathway. Within a TLPS conduc-
tive paste viahole, copper and alloy particles “micro- weld” together to form a mechanically and electrically sound connection. The re-melt tempera-
tures of the various phases of these materials are well above reflow temperatures and, for all intents and pur- poses, nonreversible in the PCB.
Bonding films not rein-
forced by glass fabric provide many advantages for HDI structures both in fabrication and signal integrity. While laser drilling is normally not per- formed on B-stage materials, it is required for the prelamination via- hole formation sequence. In glass fabric circuit materials,
Circuit materials filled with glass fibers can suffer melting of the end of the fibers, resulting in “slag” droplets.
dense, continuous interconnections that are consistent from pad to pad and via hole to via hole. The film system is not depend-
the ends of the glass fibers tend to melt from the heat of the laser, form- ing “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 uni- form composition: polymer without the glass. In any case, the laser heat source must be matched to the bond- ing 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 com- monly machined prior to lamination, they are formulated with reduced- flow characteristics. Such flow behav- ior 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 materi-
al is formed by replacing the “rubber- ized” matrix of a flexible circuit with
ent on glass fabric, thermal aging, or other factors for flow control, so the resin effect on a TLPS viahole is pre- dictable and consistent over a long shelf life. The film is a stand-alone dielectric material without the elec- trical and thickness constraints imposed by glass, but with superior thermal properties similar to a lead- free-compatible, glass-reinforced sys- tem. 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 flexi- ble-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 con- trolled z-axis expansion. Together, these materials offer producers of multilayer PCBs ease of implementa- tion 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:
www.ormetcircuits.com r
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 mate- rial system suitable for use with TLPS paste and interconnections. These thin-film circuits achieve
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