Alloy Composition
Lead-free solder paste formulated with a tin-silver-copper alloy and a specific particle size distribution represents the standard material for modern surface mount assembly. For most standard pitch boards, sac305 powder type 4 provides a reliable balance of printing resolution and cost efficiency. The alloy consists primarily of tin, combined with small percentages of silver and copper.
Its particle size distribution is constrained to the twenty to thirty-eight micron range.
Deposition Reliability
Stencil release efficiency determines the consistency of solder joint volume across dense component arrays. When printing with sac305 powder type 4, the aperture sizes are designed to comply with the five-ball rule to ensure that at least five solder particles can fit across the narrowest dimension of the opening. Using this size powder prevents stencil clogging on zero point five millimetre pitch components while maintaining sufficient metal load to prevent voiding.
If the stencil apertures are too small for the particle size, the paste clings to the aperture walls, causing starved joints.
Thermal Performance
Reflow profiling requires precise temperature controls to achieve complete melting of the alloy. The paste has a melting point of 217 degrees Celsius, meaning that reflow ovens must be configured with a peak temperature of 235 to 245 degrees Celsius. This temperature ensures the alloy forms a reliable bond without damaging the board.
