Do Your Homework -- When changing materials, methods or tools, examine the total process-impacts can be observed upstream, as well as downstream, of the change.

Q. My company recently changed from rosin activated (RA) flux to organic acid (OA) flux in our mass soldering operation. This change was to update our cleaning process from a solvent board wash to a deionized (DI) water system with no added chemistry. The vendor supplied and supported system is closed-loop, with filters, and produces no effluent. The filters are exchanged periodically and shipped to the vendor. The system requires only a tap water supply line and no local drain.

To keep the materials system simple, I also switched from RA fluxed, braided solder wick to un-fluxed, braided solder wick in the touch-up/rework area at the same time. The touch-up/rework operators report that the new solder wick does not work very well-even when using liquid OA flux from a squirt bottle as an assist. A few assemblers reported having had good results with a no- clean fluxed solder braid with a previous employer. Are there wash compatibility problems with this no-clean fluxed solder wick and my DI wash system?

A. Probably not. However, you don't mention that you are using no-clean flux to solder in the touch-up/rework area. If so, you now are creating a situation where you are using no-clean fluxed solder braid and OA flux-cored wire to operate with in your touch-up/rework area. I would find this mix of residues, from OA flux and no-clean solder braid flux, and possible product interactions more of a problem than the no clean solder braid flux residues alone.

Ignoring the previous issue, if the no-clean solder braid flux residues are not water-soluble, the residues might be mechanically flushed off and captured in the filters, as desired, or remain on the assembly as benign residues. If the residues are soluble, my previous contacts with your machine vendor indicate that they shouldn't be a problem. A bigger issue is the fact that the residues may be insoluble in straight deionized water and turn white when cleaned-a typical reaction of some no-clean flux residues cleaned in hot water without any chemistry. This type of reaction might be a problem for your inspection group and for customer perception.

Typically, customers do not like to see residues of any type-even if the residues are benign. Most no-clean fluxed solder wick is fluxed with material that does not leave visible residue at 1X. Consider cleaning before touch-up, using no-clean flux and solder wick for touch-up/rework and not cleaning again. However, this method would increase the amount of the second flux type used and could further confound your system.

As a simple, less complex alternative, add an OA liquid flux by changing to a braid with different physical characteristics. The braid pattern, wire configuration (flat or round) and opening size affect how much flux is held in the openings and how much solder can be wicked off the assembly. Copper solder braid is designed to conduct heat from the iron to the desired areas and, through capillary action, wick the solder off the assembly.

An involved physics issue exists with solder braid and how much flux can be held and be available. The size of the wire, number of wires in a bundle and the pics per inch (a measure of how many times the wire bundle crosses itself) affect how much flux is held on the wire and in openings. The amount of flux held can vary from one type of braid to another. The braid construction must act as a sponge for the flux, as well as the solder to be removed. The selection of the braid, to match the size of the iron tip, and the size of the connection being heated determine uniform and efficient heating.

Maintaining a single material system would seem to be the best way to go. Find a way to use OA fluxed braid in your touchup/rework area. The goal is an admirable one-keep the material system simple to avoid mix-ups and unwanted outcomes.

Les Hymes is the owner of Les Hymes Associates, Surprise, AZ; [phone]; e-mail: [e-mail].

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