Head In Pillow: Design Rules and Process Limits A head in pillow defect is a ball that has never fused with the solder on the pad. The ball sits on top of the paste, the paste melts and reflows into a fillet, and the ball remains solid on top of…
Burr Control at Drill Entry and Exit A drilled hole is formed by a tool that pushes through a stack of materials, and what happens at the top and the bottom of that stack decides the quality of the hole. Entry and exit material is placed to control those two…
Throughput Versus Quality Tradeoff Throughput and quality are usually described as opposing forces, and in most assembly processes the opposite is true. A process that produces defects consumes capacity in rework and reinspection, so improving quality increases the number of good units the line can deliver. The tradeoff is real…
PCB Strain Gauge Monitoring Board flexure is one of the few assembly stresses that can be measured directly, and measuring it changes what a factory believes about its own processes. A strain gauge bonded to the board surface records the deformation as the panel is routed, broken, handled, pressed into…
Solder Paste Warm Up and Shelf Life Rules in Practice Solder paste is a perishable material. It is stored cold to slow the chemical reactions that thicken it, brought to room temperature before use so that it prints predictably, and discarded once its working life has expired. Skip any of…
Flex PCB Stiffener Design and Placement A flexible circuit is only useful where it bends, and it is only reliable where it does not. Component areas, connector tails and anything that has to be soldered to a rigid board need to be flat and stable. A stiffener is the local…
Conformal Coating Masking Methods and Materials Conformal coating covers what it is applied to, and some features must remain uncovered: connectors that have to mate, test points that have to be probed, and components that have to dissipate heat. Masking is the process that keeps the coating off them. What…
Thermal Relief Design for Through-Hole Pads A thermal relief is a pad that is connected to a plane by a small number of narrow spokes rather than by a full connection. It exists so that a through hole joint can be soldered without the plane drawing the heat away, and…
Solder Beading on Chip Components After Reflow A solder bead is a small sphere of alloy that has formed beside a chip component, usually attached to the termination or to the mask. It is a defect because it can move during later handling, because it can bridge to a neighbouring…
Solderability and Storage Conditions A board that soldered perfectly last month may not solder today, and nothing about the process has changed. What changed was the surface. Solderability decays with time and with the conditions the board has been stored in, and the decay is invisible until the first defect…
