SMT007 Magazine

SMT-Sept2017

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26 SMT Magazine • September 2017 REWORK AND RELIABILITY: LESS IS MORE! allows small currents to flow between conductors at different voltages, reducing the surface insula- tion resistance (SIR). In some cases, the small current may cause electrolytic corrosion, which is the dissolution of metal from positively biased conductors. This rarely will cause a conductor to be complete- ly eaten a w a y a n d b e c o m e an open cir- cuit. It also can provide metal cations in the electrolyte film. Now comes the worse and most destructive failure, electro- chemical migration (ECM). Those met- al ions in the electrolyte film will migrate to the cathodic, or negatively bias conductor because of the electrical field between the two conduc- tors. When the metal cation gets to the cath- ode, it can pick up some electrons and deposit on the conductor. This deposition will usually occur at high energy locations, causing the for- mation of tree-like structures called dendrites. These dendrites will grow all the way to the op- posing conductor causing a short circuit. These dendrites are delicate and can be destroyed by the resistive heating current flowing though the thin conductor. Noise results from the repeated forming and breaking of the dendrites, as seen in Figure 1. Only J-STD-004B type -L0 and -L1 fluxes should be used for no-clean rework; fluxes that contain some rosin or a similar encapsulating resin, such as ROL0 and REL1, are even better. The rosin or resin in the flux will harden and help trap activators that weren't completely heat deactivated. Some circuit assemblers clean reworked ar- eas with isopropyl alcohol or another sol- vent. For no-clean material rework, this is usu- ally done for cosmetic reasons and more like- ly to hurt the electrochemical reliability than help it. No-clean fluxes are designed to be left on the circuit board. Spot cleaning may only move the dirt around. For circuits boards that are usually cleaned, spot cleaning reworked ar- eas are not recommended. The board should be reworked with washable fluxes, and it should be put through the usual washing process again to ensure all rework fluxes are adequately removed from the board. This is especially important if the finished circuit is to be conformally coated, as coating over some flux residues can be worse than not coating at all. In conclusion, be careful not to leave exces- sive or improperly heated flux residues after re- working circuit boards, or you might have more problems. SMT Karen Tellefsen is a senior research chemist, R&D, for Alpha Assembly Solutions.

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