How cleaning a no-clean flux causes leakage and corrosion: take care not to leave moisture-absorbing conductive residues.Cleaning flux residues after reflow (1) The process of heating and melting the solder that has been screen printed onto a printed circuit board in order to bond chips and other components to the board. Surface mount chips (SMT) use the reflow method. Contrast with wave soldering. See also reflowable text. and wave soldering Applying liquid solder to the underside of printed circuit boards in order to bond the chips and discrete components that are placed on top of the board and whose metal leads (pins) extend through the board. is not the norm today. Cleaning has been left to high-reliability manufacturers to make water- or solvent-based systems work. However, much of the industry has chosen not to clean after soldering, instead opting for low-solids fluxes designed to be left on and benign when properly heat-activated. Most companies have qualified and worked through the issues to make these assembly techniques work; in our estimation, around 70% of assemblers do not clean. But what happens when a no-clean assembly process is cleaned? Here we discuss the results of three different cleaning approaches on a no-clean flux with paste and reflow, liquid spray flux and wave-soldered assemblies. [ILLUSTRATION OMITTED] [TEXT NOT REPRODUCIBLE IN ASCII ASCII or American Standard Code for Information Interchange, a set of codes used to represent letters, numbers, a few symbols, and control characters. Originally designed for teletype operations, it has found wide application in computers. ] Condition 1. Cleaning a mixed technology no-clean flux with only water in a new inline cleaner with high-pressure sprays and closed-loop DI systems. Condition 2. Cleaning a mixed technology no-clean flux with saponifier sa·pon·i·fy v. sa·pon·i·fied, sa·pon·i·fy·ing, sa·pon·i·fies v.tr. 1. To convert (an ester) by saponification. 2. To convert (a fat or oil) into soap. v.intr. added to the wash section at 10% using high-pressure sprays. Condition 3. Cleaning a mixed technology no-clean flux with saponifier, followed by DI water steam (manual) and low-pressure sprays. Some no-clean fluxes are more water-cleanable than others, but this is typically in open areas; the fluxes do not solubilize sol·u·bi·lize v. To make substances such as fats soluble in water by the action of a detergent or similar agent. or are not rinsed well in areas of entrapment entrapment, in law, the instigation of a crime in the attempt to obtain cause for a criminal prosecution. Situations in which a government operative merely provides the occasion for the commission of a criminal act (e.g. (below components and in vias). White residue pockets after water cleaning and drying indicate that flux has not been completely removed and has reacted with flux on the assembly surface. Even completely dry, this white residue can absorb moisture in the field. This can cause leakage and stray voltage stray voltage accumulation of low voltages in the metalwork of a milking parlor due either to leakage from poor wiring or to poor earthing (grounding) with no outlet for static electricity. Very small voltages cause restlessness and a fall in milk yield. problems. Our analytical approach to evaluate the differences in the conditions listed above is a localized extraction system and ion chromatography analysis, along with elevated humidity testing with an electrical biased state applied to the assemblies. * Condition 1 assemblies failed electrically when exposed to high humidity and powered within 48 hrs. at 85% RH at 65[degrees]C. * Condition 2 assemblies failed electrically when exposed to the high humidity and powered within 72 hrs. at 85% RH at 65[degrees]C. * Condition 3 assemblies passed 500 hrs. at 85% RH and 65[degrees]C. Table 1 (online) shows the ionic differences between the different process groups. The level of chloride, bromide bromide, any of a group of compounds that contain bromine and a more electropositive element or radical. Bromides are formed by the reaction of bromine or a bromide with another substance; they are widely distributed in nature. , nitrate and sodium for all three assemblies is well below our recommended limits. The failed Condition 1 is very high for WOA WOA Wacken Open Air (music festival) WOA Work of Art WOA Western Orthopaedic Association WOA Web Offset Association (Nashville, TN) WOA World Airways, Inc (ICAO code) (Figure 1). The failed Condition 2 is high in WOA and ammonia (from the saponifier) and the residue is conductive and corrosive (Figure 2). The good Condition 3 assembly showed great electrical performance for 500 hrs. of humidity exposure (Figure 3). The use of a saponifier, steam rinsing with DI water, followed by DI water rinsing did a good job of solubilizing the flux. Cleaning a no-clean flux with water or just soap can leave moisture-absorbing conductive residues in areas of entrapment, if not properly removed. [FIGURE 1 OMITTED] [FIGURE 2 OMITTED] [FIGURE 3 OMITTED] Terry Munson is with Foresite Inc. (residues.com); tm_foresite@residues.com. His column appears monthly. |
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