13.10 Solder Pastes for SMDS

Solder pastes, often called solder creams, are used in "reflow-soldering" of surface mount components, where the application of solder and the heat supply are separate steps in the fabrication process. A solder paste basically consists of solder in powder form and a flux with some additives necessary to produce the desired behaviour of the paste during or after its application.

Solder pastes are applied either by stencil or by screen-printing method. Pick-and-place throughput is an important issue in deciding the type of solder paste dispensing system. Erdmann (1991) brings out the stenciling technique including stencil development, stencil cleaning and printing etc. He points out that the demands of stencil are more rigid than that of ordinary SMT screen printing and stenciling, particularly for maintaining near perfect registration.

Large pick-and-place systems require volumes that only screen printers can provide. Pick-and-place throughput in the range of 1500 to 3000 components per hour, however, is ideal for today's dispensing equipment, which can produce 16000 dots per hour in a typical production environment (Cavallaro and Marchitto, 1991).

A rotary positive displacement programmable pump as shown in Figure 13.23(a) can be effectively used for solder paste dispensing involving high speed application of a large number of very small dots. The pump is driven by a DC motor. An electro-magnetic clutch engages and disengages the Archimedian screw. Mounted above the screw is a bellows coupling that aligns the clutch and lead screw and reduces the impact of the Z-axis sensor by...