Assembly of multi-chip modules using eutectic solders

The present invention relates to the soldering of different types of chips to multi-chip module substrates and more particularly to joining to the same substrate of both chips requiring high temperature reflow and materials and/or techniques and those requiring low temperature reflow materials and/or techniques. Chips requiring high temperature reflow for attachment to a module substrate are attached first and then a eutectic water soluble solder paste and/or water soluble flux is dispensed on both the TSOP and the PBGA chip pads instead of using the paste screening techniques. The dispensing is done by injecting solder on the solder sites individually. Characteristics of the solder paste used is that it must be fluid enough to be injected onto the individual sites yet have enough body that it remains in place and does not run from site to site once dispensed. A paste capable of providing such characteristics is one having: a ) a very fine particle size in the range of 400 to 500 mesh and preferably between 400 and 450 mesh; b) a low viscosity (below 500 k centerpoise and preferably between 425 to 375 cps); and c) a solid content of 86% or lower and preferably between 84 and 80%. The chips to be joined to the module using low temperature module joining are then placed on the module substrate surface and then put in an oven for reflow to attach these chips to the surface. After the joining process, the modules are cleaned by spraying deionized water on them to remove any flux or solder residues and the modules are electrically tested to determine if the joining process has resulted in connection with the proper mechanical and electrical characteristics.