Functional Test Fixtures Owned by the Factory or the Buyer
Buyer-owned test fixtures require explicit PO line items, complete source files, and calibrated limit audits to prevent factory lock-in.
Solder Flux Contamination
Process Impurity
Chemical residues left on a circuit board surface after soldering can lead to long-term reliability failures. Solder flux contamination occurs when the cleaning process fails to remove the acidic or ionic byproducts of the reflow stage. These residues can attract moisture from the air and create conductive paths where none should exist.
This condition is measured during the cleanliness test and stops being a concern once the board passes a de-ionized water wash.
Corrosion Potential
The presence of these chemicals is particularly dangerous in high-humidity environments. Under the influence of solder flux contamination, the copper traces on the board can begin to corrode and grow metallic dendrites. These tiny whiskers can bridge the gap between two pads and cause a short circuit that kills the device.
Manufacturers of high-reliability electronics often use no-clean fluxes that are designed to be safe if left on the board. However, even these materials can cause problems if they are not fully activated by the heat of the oven. Regular testing using the resistivity of solvent extract method helps the factory monitor the effectiveness of their washing systems.
Cleaning Efficacy
Removing all traces of the flux is necessary for boards that will be coated with a protective resin. Solder flux contamination prevents the coating from sticking to the board surface and can trap corrosive agents underneath. This leads to a higher risk of field failure in harsh conditions.