Stencil Foil Material and Coating The stencil foil is a thin sheet of metal with a pattern of apertures cut into it, and its material and coating determine how the paste releases, how long it lasts and how it behaves when it is cleaned. The choice is narrow and the…

Component Substitution Approval A substitute component is proposed when the original is unavailable, more expensive or discontinued, and the decision is usually made under schedule pressure. The approval is what keeps a procurement decision from becoming a design change without an owner. Equivalence depends on the function the part performs…

Impedance Coupon Design and Measurement An impedance coupon is a small pattern on the production panel that repeats the controlled lines of the product, so that the impedance can be measured without cutting the product apart. Its value depends entirely on how faithfully it repeats the product, and that is…

PCB Etch Compensation and Finished Line Width The copper that is imaged on the panel is not the copper that arrives at the customer. Etching undercuts the resist, plating adds thickness to the walls and the surface, and the finished line comes out narrower than the artwork. Etch compensation is…

Board Handling and Packaging After Reflow A board that has been through reflow is a finished assembly, and everything that happens to it afterwards either preserves or degrades it. Handling and packaging are the last chance to do that well. Where the Damage Comes From Flexing during handling, contact with…

Selective Soldering Process Window and Nozzles Selective soldering places molten alloy on individual joints rather than passing the whole board through a wave. It exists because some assemblies cannot take a wave, and its process window is narrower than the wave it replaces, which is why it is usually the…

Assembly Work Instructions and Operator Training A work instruction tells the operator what to do at one station. It works when it is written for that station rather than for the product as a whole, and when it changes at the same moment the product does. What Belongs on the…

Selective Soldering Nozzle Selection for Through Hole Assembly Selective soldering solders the through hole joints of a board that also carries surface mount devices, using a small nozzle of flowing alloy rather than a full wave. It is selected when a connector or terminal block cannot survive a second full…

Nitrogen: Preparation, Placement and Process Control A reflow oven filled with nitrogen reduces the oxygen available to the joint while it is molten. The consequence is better wetting and less oxidation, and the size of the benefit depends on the paste and on the joint being formed. What the Oxygen…

Alternate Part Qualification and Change Control An alternate part is a second source for a component that is already qualified, and qualifying it is a process activity rather than a purchasing one. The two parts have to be equivalent in the ways that matter to the assembly and to the…