It may seem obvious that we need dimensions on an engineering drawing to specify a solder preform, but we often receive drawings with missing dimensions or tolerances. I’m guilty of that too – especially with more complex drawings. Missing dimensions can be resolved, but checking for this before you send the drawing can save time.
Tolerances, on the other hand, are generally a bit more of an issue to resolve. It is typical in all areas of manufacturing for machinists to desire wider tolerances to optimize production, but the question of “what tolerance is required” is tough to answer – especially for new builds. Sometimes it may not even be obvious what the tolerance constraints are. If there are no tolerances available, we leave the guesswork to the customer, although we offer a set of standard tolerances as a suggestion. If the tolerances aren’t critical, it’s usually pretty safe to include a universal +/-0.005” tolerance note on the print.
Omitting units of measure may be the most frequent error we find on solder preform drawings. There can be no guessing here – since the dimensions in millimeters are grossly different than those same dimensions in inches. Again, a simple note “Unless otherwise specified, dimension are in inches” (or mm) can be quite helpful. Mixed units should be avoided, but can be used if there is a need for them.
One thing to note about pure indium preforms: tight tolerances are often not needed. In compression, pure indium is so soft that it will slightly elongate in the x and y axis as it is compressed in the z axis. During reflow, the pure indium will be flowing even more. Slight variations in length, width, or thickness will not make much of a difference in the final assembly.
This post is part of the “Complete Intro to Pure Indium Preforms” series