14 February 2001Manufacturing / Production Technology, Hardware & Services
With electronic assemblies becoming increasingly dense, and packages and substrates smaller than ever, electronics manufacturers are looking for the ultimate in precision material deposition. Zetech explains more about dispensing.
Accurate small dot dispensing (6-10 mil dots) has become a necessity for DCA (direct chip attach), multichip module and high density interconnect for both solder paste and conductive epoxy.
Maintaining material properties throughout the dispensing process is of paramount importance. This is accomplished by monitoring both pump RPMs and air pressure. RPMs are held to tight specifications using a closed-loop brushless DC servo motor. Speed, indexing rate and acceleration/deceleration are programmable so that optimal dispensing speeds can be maintained for each application.
Material feed is controlled by carefully maintaining constant feed with minimum compression of material. As a rule of thumb, solder paste requires 4-12 PSI while conductive epoxy requires 1-5 PSI. The pressure setting is material-dependent.
For most packaging requirements, placement accuracy of 1 mil or better is required to accommodate the tight tolerances of highly populated assemblies. A touch probe is used to sense the height of the substrate and to establish the proper dispense height. A footed needle further improves Z-height consistency for substrate surface imperfections. The dispense tip is a one-piece construction with smooth interior surfaces for enhanced material flow.
The dispense tip is conically chamfered to reduce surface tension between the needle and the material. In contrast, conventional dispensing tips are not conically chamfered and are, in most cases, two-piece construction using rolled tubing which hinders small dot dispensing because of poor interior surface characteristics.
Dispensing
GPD, a company represented in South Africa by Zetech, manufactures the DS series of liquid dispensing equipment, ideal for low volume solder dot and conductive epoxy applications. The systems offer less than 1 mil location accuracy in the dispense area using a traceable glass plate procedure. Software maps the exact location of each gantry position and automatically adjusts the system for inaccuracies. Z-height repeatability is 0,2 mils (with footed needle) which allows for precise needle positioning relative to the dispense pattern.
The GPD micro valve is specifically designed for small volume dispensing. The pump cartridge is available in three different styles: floating cartridge for custom-footed needles; fixed cartridge for custom nonfooted needles; and a Luer-Lok style cartridge for disposable Luer-Lok needles. A conical needle tip facilitates material release during dispense. Cartridge and needle can be easily extracted and stored together overnight.
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