Deposition of Molten Eutectic Solder using Jet Printing Techniques
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MICHAEL D. SNYDER AND RONALD LASKY Universal Instruments Corporation, P.O. Box 825, Binghamton, NY 13902-0825
ABSTRACT
This paper discusses the use of Ink Jet printing techniques to dispense small (50 to 75 micrometer diameter) particles of molten eutectic solder individually at programmable dispense rates from drop on demand to several thousand per second. Alternative jet dispensing techniques are discussed. The technology could allow the selective application of programmable amounts of solder on precision circuit boards and wafer substrates, while avoiding the high cost and flexibility limits associated with hard tooling. Large solder features can be constructed by dispensing individual droplets and relying on surface tension to draw them together to form a large single feature. Alternatively, columnar features can be created by successively dispensing solder droplets at the same site, allowing time between successive droplets to avoid forming a single large spherical feature. Several potential application areas in industry are discussed along with some of the issues associated with the projected performance of the method in the accuracy and speed domains.
INTRODUCTION
Modern electronics packaging and assembly technology is being very strongly driven by the desire for an increased number of miniaturized and portable final products. This trend is reflected in the popularity of products such as Pagers, Personal Digital Assistants, PCMCIA format cards and the like. This dramatic increase in miniaturization has caused pressure to increase the space efficiency of both first and second level electronics packages. One manifestation of this trend is the movement to fine lead pitch devices such as TAB componentry, as well as an increase in the use of flip chip and unpackaged die devices. In order for such devices to be successfully applied, the amount of solder forming the electrical interconnection between the device and the circuit substrate assumes a critical degree of importance. Too much, and the solder can short between successive interconnection points, some as close as 250 micrometer pitch. Too little, and there will not be enough solder to assure all interconnection points are making electrical contact with the circuit substrate. The problem of applying just enough solder to the device or substrate to provide these high density interconnections has been addressed using several technologies. Screen printing, for 201
Mat. Res. Soc. Symp. Proc. Vol. 390 ©1995 Materials Research Society
example has been used to deposit solder paste, which on subsequent reflow leaves solder deposits on circuit interconnect pads. Other techniques such as electroplating can also leave precise solder deposits on the board. Each of the technologies have, however, certain drawbacks. Screen printing for example can be prone to reliability problems as the pitch falls below 150 to 200 micrometers, while electroplating can prove to be a costly solution. In addition, to use these technologies, both hard tooling and increased process step
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