|Power semiconductor is typical of devices that need re-tinning. |
In the years since RoHS took effect, component manufacturers have converted virtually all of their components to lead-free solder compatibility, leaving those with exemptions, e.g., Hi-rel and military electronics suppliers, with no source for components for tin-lead soldered assemblies.
To remedy this, companies have taken to hot solder dipping, a process, whereby aged components are refurbished in preparation for re-qualification to "High Reliability" standards. The purpose is to remove the pure tin finish from the component terminations, and refinish with tin-lead solder. All of the pure tin must be removed to avoid tin whisker growth. To do this, the component's terminations must be dipped in hot tin-lead solder right up to the body of the component.
Published research has shown that the only reliable way to mitigate tin whiskers and prevent their growth is to dip the leads in molten alloy. This creates a "fused" intermetallic finish that is unlike the non-fused electroplated finishes, which are a lot like a coating of sand — not fused or connected and prone to tin whisker development under certain conditions.
In addition to removing tin, we have the need to ensure that older or "legacy" components are solderable. These components may have been inventoried for years under uncertain conditions, making the solderability of the terminations questionable. The molten solder dipping process will remove the old finish while applying a fresh "fused" SnPb finish.
Dipping means that the body of the part may be thermally stressed. The whole operation, i.e., fluxing, dipping, and cleaning, must be precisely controlled not only to ensure repeatable results, but also to prevent thermally-induced internal damage to the component. If the external terminations become too hot for too long, heat transferred into the body can cause lead fingers to delaminate from the mold compound, setting the stage for a future electrical failure. The heat may also cause a crack in the mold compound itself. The duration of the hot solder dip needs to be precise, and the immersion depth of the terminations also needs to be precise. Also, leaving even a small band of the original pure tin solder on a termination opens the way for tin whisker growth in the field. The only way to achieve consistent quality and reliable results is through a controlled, automated process. This ensures controlled dwell times, depths, fluxing, and other process variables to ensure repeatability.
Two Solder Pots
In simplest terms, an automated lead tinning system consists basically of a central fluxing station with 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.
|Component leads are handled and tinned automatically, keeping tight temperature control. |
The system works in conjunction with pallets that hold the components in a known position through the process. Under program control, a pallet of components moves to the central flux station where the component 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 solder pot for the final dip of fresh alloy creating a homogeneous intermetallic 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 compliance. 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 ample process control, it is possible to create and add programmable routines 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 enhance the quality of the final lead finish.
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 components 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 production. The wetting balance test will graphically demonstrate the best flux that is compatible with the component lead finish. As an example, just arbitrarily using rosin flux on a legacy lead will likely not provide a satisfactory wetting curve, while an active flux will. With the flux chemistry in check the application of the flux must be accurate 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 activate 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 temperature rate of rise. Pre-heating is instrumental in preventing thermally-induced internal component damage as described earlier.
Depending on the type of flux used (typically RMA for Military products, or water-soluble fluxes for most others), cleaning will follow, typically batch-type. This is followed by:
- Visual inspection.
- Final wetting balance test to ensure best solderability.
- Lot report documenting the details of the process.
Some other process tips:
- The time that each side of a component remains in the hot solder dip should not exceed 5 seconds.
- Cleaning, if necessary, should occur shortly after processing (in under an hour, at most) and promptly after cool-down to proper temperature, to ensure easy cleaning and complete removal of all residues.
- An automated process with strict control helps ensure that components will be processed in accordance with IPC/EIA JSTD-001 and ANSI-GEIA-STD-0006.
- Inspect afterwards visually or with other types of imaging, e.g., X-ray, as deemed necessary.
- QFPs and other gull wing leaded fine pitch components can also be tinned; we have successfully processed gold leaded devices with lead pitch centers of .020-in. (.5mm) and toe to toe of 3-in. (75mm).
Contact: A.C.E. Production Technologies, 3010 N. First Street, Spokane Valley, WA 99216 509-924-4898 fax: 509-533-1299 E-mail: firstname.lastname@example.org Web: http://www.ace-protech.com