BGA Rework with Local Heating A ball grid array is removed and replaced with local heat, and the difficulty is that the joint is underneath the package and the heat has to pass through the package to reach it. Every parameter is a compromise between reaching the joint and protecting…

Zinc Whisker Risk in Tin Plated Components and Boards A whisker is a thin metal filament that grows out of a plated surface over months or years, and when it bridges two conductors it creates a short that is almost impossible to predict. Whiskers are a well known hazard in…

Creepage and Clearance Rules on Printed Circuit Boards Creepage and clearance are the two distances that keep a board from arcing, and they are among the most frequently missed requirements in a design review. They are not the same thing, they are not interchangeable with conductor width, and they depend…

Drill Machine Accuracy and Spindle Maintenance A drilling machine places a hole within a tolerance of the design position, and that tolerance is one of the inputs to the annular ring. The accuracy depends on the spindle, the machine calibration and the material handling, and each of the three is…

Grain Structure and Alloy Additives in Solder A small addition of a fourth element to a solder alloy changes the grain structure that forms when the joint solidifies, and the change is visible in the reliability rather than in the melting point. The additive is a process decision with a…

Panel Fiducial Design Fiducials are the marks a placement machine looks for to decide where the board actually is. They are small, cheap and completely unforgiving: if the vision system cannot find them reliably, the machine will place every part with an offset that no amount of program tuning can…

Fabrication Notes and Drawing Requirements Fabrication notes are the sentences on a drawing that carry the requirements the graphics cannot. They are short, they are read by the shop before the panel is made, and a missing one moves a decision from the designer to the process. What the Graphics…

Selective Soldering: Preparation, Placement and Process Control Selective soldering solders the through hole parts on a board that has already been reflowed, one joint or one group of joints at a time. The nozzle is the interface between the machine and the joint, and it decides how much heat arrives,…

Stencil Tension and Its Effect on Print Quality A stencil is a sheet of metal held in tension across a frame, and the tension is what keeps it flat when the squeegee passes over it. A stencil that has lost tension prints a deposit that varies with the position on…

Solder Ball Removal and Acceptance Criteria A solder ball is a small sphere of alloy on the board surface that is not part of a joint, and the question it raises is whether it can move. A ball that is held by the mask is a cosmetic defect and one…