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52 SMT Magazine • August 2017 into the solder paste, and completely through the through-holes. Solder materials suppliers have been instrumental in making these process ad- vances viable by developing solder pastes with rheological properties optimized for this type of processing, increasing process yields and reducing issues such as paste dripping from the pins as the assembly moves down the SMT line, particularly in the reflow oven. Solder material suppliers have also been pro- active in developing materials and process solu- tions to some of the challenges that are inher- ent to the PiP process. One such challenge is the need to provide enough solder volume to com- pletely fill the through-hole providing the re- quired mechanical and electrical reliability. In many cases, due to restricted real estate on the surface of the circuit board and the intricacies of component design, it is not possible to over- print with enough solder paste (which is only about 50% solder metal to begin with) to com- pletely fill the through-hole, especially on the inner rows of a fine pitch connector. As board thickness increases, it becomes even more challenging to provide enough sol- der to fill the through-hole. One solution that has been developed that eliminates the need for a step stencil, is the use of solder preforms to provide the additional solder to the joint. Sol- der preforms can be stamped in various shapes from solder ribbon and are pure metal alloy. The volume of solder required to fill a through- hole can be calculated and the appropriate sized preform can be stamped to provide enough sol- der to fill the through-hole. Preforms packaged in tape and reel can then be fed into pick and place equipment to be automatically placed in the appropriate locations. PROCESS STEP ELIMINATION