Solderable Finish Shelf Life and Storage A solderable finish is a thin layer of metal that has to remain wettable until the moment it is soldered. It oxidises, it corrodes, it diffuses into the layers beneath it, and it picks up contamination from the environment. Shelf life is the period…

Reflow Atmosphere Oxygen Level Control Reflowing in nitrogen reduces the oxygen around the joint, which reduces the oxidation of the alloy and the pad during the melting phase. The result is better wetting, a wider process window and, in some cases, a lower defect rate. The oxygen level required depends…

Component Orientation and Polarity Verification An orientation error is a defect that passes almost every test. A diode placed the wrong way round conducts in the reverse direction and may still pass a functional test that only checks for presence; an electrolytic capacitor reversed may work briefly and fail later;…

Solder Joint Cooling Rate and Grain Structure A solder joint is not a homogeneous piece of metal. It is a composite of tin-rich grains, particles of intermetallic compound and the layers that form where the alloy meets the copper or the component finish. The size and distribution of those features…

Warpage Measurement: Design Rules and Process Limits Warpage is measured as a deviation from a reference plane, and the reference plane is the part of the measurement that is most often left undefined. Two inspectors measuring the same board with different supports will report different numbers, and both will be…

Wash Bath: Cleaning Bath Monitoring and Contamination Control A cleaning bath is a chemical process that changes as it is used. Flux dissolves into it, the active chemistry is consumed, the rinse water concentrates and particles accumulate. The bath that cleaned perfectly on Monday may be leaving residue by Friday,…

Design for Test Coverage Analysis Test coverage is the proportion of the possible defects on a board that a set of tests can actually detect. It is not the same as the number of tests, and it is not the same as the yield. A board with a hundred test…

Functional Test Fixture Design and Upkeep A functional test fixture holds the assembly, connects it to the test equipment, and lets the test programme exercise the product the way it will be used. It is a piece of equipment in its own right, with an alignment tolerance, a wear life…

Solder Joint Volume and Pad Geometry Solder joint volume is one of the few characteristics of an assembly that is decided at the design stage and then reproduced, or not, by the process. The land pattern sets how much solder the joint can hold, the paste deposit sets how much…

Voiding: Preparation, Placement and Process Control Voids under a QFN thermal pad are the most common measured defect on modern assemblies, and they are also the one most often reported without context, because the number depends on how it was measured. The mechanism is straightforward and the corrections are known,…