This chapter addresses the surface mount technology (SMT) board assembly process for reflow soldering SMT components to boards, as well as rework soldering for removing and replacing individual components on already-assembled boards.
The information in this document is for reference only. Manufacturing processes are unique, and may require unique solutions to ensure acceptable levels of quality, reliability, and manufacturing yield. Due to differences in equipment and materials, customer-specific process parameter development and validation is required.
9.2 Solder Paste Printing
Standard tin-lead (SnPb) solder paste alloy is composed of 63% tin (by weight) and 37% lead, which is commonly expressed as 63Sn/37Pb or Sn/37Pb, a eutectic composition that melts at 183C.
Although there are a number of lead free (Pb-free) alloys, the most commonly used compositions contain tin, silver, and copper, commonly expressed as SAC, for SnAgCu. Within SAC solders, by far the most common usage is Sn/3Ag/0.5Cu, a near-eutectic which melts between 217°C and 220C.
Apertures sizes can be 1:1 with pad size, but certain parts may require reduced apertures to reduce solder ball defects. Larger pads may benefit from crosshatched openings, to reduce the amount of paste applied, and to control scavenging.
Pb-free solder paste may spread less during reflow than SnPb paste, potentially leaving extremities of pads unsoldered. Although full pad coverage by solder is not a requirement of IPC-A-610, some customers prefer to enlarge apertures to ensure that pads are covered.