February, 2013
www.us-tech.com
Hot Solder Dip Component Lead Refurbishing
Continued from page 73
two (2) solder pots, one on each side of the fluxing station. One solder pot is usually dedicated to flushing off the original coating (or plating) while the other solder pot is dedicated to the “virgin” alloy for the final coating. The system works in conjunc-
tion with pallets that hold the com- ponents in a known position through the process. Under program control, a pallet of components moves to the central flux station where the compo- nent leads are immersed to a specific depth. The pallet then moves to the first solder pot (scavenging pot) where the component leads are dipped into the solder alloy and “scrubbed” to remove the existing coating. The pallet returns to the flux station where the leads are once again fluxed then to the second sol- der pot for the final dip of fresh alloy creating a homogeneous intermetal- lic coating. Use of a versatile pallet holder and pallets that accept a wide variety of components (e.g., axials, connectors, DIPs, QFPs and virtually all leaded devices) results in minimal change-over time. Conversely, when fitted with
two “lead free” solder pots, the same system can be used to convert tin- lead plated components to RoHs com-
Two “lead free” solder pots let the same system be used to convert tin-lead plated components to RoHs compliance.
pliance. In this process, the original lead-bearing plating is dissolved into the sacrificial alloy of the first pot. The component leads are re-fluxed followed by dipping into the second 2nd “virgin alloy” solder pot for the final lead-free coating. In an automated system with
am ple process control, it is possible to create and add programmable rou- tines such as “agitate in the solder” to help remove the original coating, or to specify and control “withdrawal rate” from the final solder bath to increase the solder thickness. This gives the process engineer the ability to en - hance the quality of the final lead fin- ish.
Use the Right Flux The flux choice is critical to
match the activity needed to remove the old finish and to match the best “wetting” to the base metal. This can vary considerably with aged compo- nents so it is wise to use a wetting balance tester to determine these requirements just as it is wise to test the fluxes before jumping into pro- duction. The wetting balance test will graphically demonstrate the best flux that is compatible with the com- ponent lead finish. As an example, just arbitrarily
using rosin flux on a legacy lead will likely not provide a satisfactory wet- ting curve, while an active flux will. With the flux chemistry in check the application of the flux must be accu- rate and repeatable. Too much flux will cause unwanted residue on the component and contamination of the fixtures. If the fluxing is done just right,
there should be little or no flux residue left but the solder finish should be bright and smooth. Pre-heating is necessary to acti-
Component leads are handled and tinned automatically, keeping tight temperature control.
vate the flux. For most components this can be achieved by hovering slightly above the solder pot. For heat sensitive components it is best to hover over a forced hot air station to ramp the body temperature to a safe level to prevent thermal shock. Both methods should be thermally profiled to ensure the correct temper- ature rate of rise. Pre-heating is instrumental in preventing thermal- ly-induced internal component dam- age as described earlier.
Continued on page 78
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