Solder replacements

ED resists have been used for the protection of circuit pattern tracks and plated through-holes during subtractive etching of copper in the manufacture of printed circuit boards [15, 25]. This involves the deposition of a polymeric etch-resist in place of the traditional metallic tin or tin-lead solder, providing a faster, simpler and more environmentally acceptable process that eliminates the need for disposal of toxic wastes from the plating and stripping chemistries used. 

The coating is applied to printed circuit board outerlayers to which aqueous-developable dry film has been previously laminated, imaged and developed. The ED film readily withstands the subsequent dry-film stripping and copper-etching operations before being itself removed. An aqueous-developable dry film is necessary so that the ED resist is not removed at the same time as the dry film. 

These coatings are usually non-photoimagable. However, a photosystem can be incorporated to improve performance if required [15]. 

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Chung K, Ozguz V. Flexible conductive adhesive as solder replacement in flip chip interconnection. In Proc. Semicon. West. 1998 127-131. 

Adhesives used for screen or stencil printing in surface-mount applications are generally electrically insulative types whose functions are mechanical attachment and thermal dissipation. However, electrically conductive, silver-filled epoxies have been used for many years as ohmic contact adhesives to interconnect bare-chip devices in hybrid microcircuits and are used as solder replacements for surface mounting of components on printed-circuit boards. Regardless of their... 

Table 5.7 Compatibility of various solder replacement adhesives with surfaces and printing resolutions...

Table 5.8 Properties of adhesives used as solder replacements... 

S. Stability of solder replacements on non-noble metal surfaces 312... 

The unstable behavior of some solder-replacement adhesives has been attributed to galvanic corrosion. Similar to most corrosion mechanisms, condensed or absorbed moisture on the surface and dissimilar metals are required to form a galvanic cell. The silver filler acts as a cathode while the substrate metallization acts as an anode and is oxidized. In the case of tin-lead solder surfaces, the solder, which has a lower electrochemical potential (0.13 V) than silver (0.79 V), becomes the anode at which corrosion and oxidation occur. A smaller potential difference between a copper surface and silver accounts for some improvement in contact resistance over the solder-silver couple. 

To minimize the corrosion reactions, oxygen scavengers and corrosion inhibitors may be used as additives in solder-replacement adhesives. The percent changes in... 

Table 6.3 Behavior of solder-replacement adhesives on non-noble metal surfaces ... 

Zwohnski M, Hickman J, Rubon H, Zaks Y. Electrically conductive adhesives for surface mount solder replacement. In Proc. Second Int. Conf. Adhesive Joining Coating Tech, in Electronics Mfg. Stockholm Sweden 1996 333-340. 

Electrically conductive adhesives that are used as solder replacements are evaluated for material characteristics and contact resistance on solder surfaces. In one study, the requirement for volume resistivity is given as 0.001 Q-cm or less and for contact-resistance change as 20% or less after 500 hours of 85/85-exposure. ... 

Chung, K., and Ozguz, V., Flexible Conductive Adhesive as Solder Replacement in Flip Chip Interconnection, Proc. Semicon. West 1998, pp. 127-131(1998)... 

Table 5.7. Compatability of Various Solder Replacement Adhesives with Surfaces and Printing Resolutions...

Solder Replacement/ Supplier Compatability with Surface Finish Printing Resolution ... 

Stability of solder replacements on non-noble metal surfaces. A new area of concern for electrical stability arises because of the increasing use of conductive adhesives as replacements for solder. Some conductive adhesives show unstable electrical-contact resistance when used on nonnoble metal surfaces such as copper or tin-lead solder. Although stable on gold, palladium, platinum, and silver surfaces, the same adhesives were found to be unstable on tin, tin-lead, copper, and nickel surfaces. The... 

The reliability of several silver-filled epoxies as solder replacements has been studied and reported by several investigators.In one study, adhesives formulated with an epoxy resin (Epon 862 ) and cured with... 

Table 6.2. Behavior of Solder Replacement Adhesives on Non-noble Metal Surfaces l... 

Zwolinski, M., Hickman, J., Rubon, H., and Zaks, Y., Electrically Conductive Adhesives for Surface Mount Solder Replacement, Proc. 2 Inti. Conf. on Adhesive Joining Coating Tech. In Electronics Mfg., Stockholm Sweden, pp. 333-340 (1996)... 

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