Two complaints about shield cans after reflow are common: surface discoloration (yellowing, darkening) and one corner lifting. For discoloration, first tell apart whether the plating itself discolored from heat or flux residue turned yellow; for lifting, the cause is not necessarily too little solder — also check the flatness of the shield can itself and the heat from laser marking. In the cases we read, for the same shield can, the “tin-plated” version had 100% discoloration and the “unplated” version 0%; and lifting on cans that were marked without added solder disappeared once solder was added.
For how to choose a shield can (one-piece, two-piece, material), see “How to Choose a Shield Can“; for how to judge component leads that discolor after reflow, see “How to Judge Component Lead Discoloration After Reflow“. The figures in this article come from production-line documents we read (with company names, customer names, and part numbers removed); they are values from single documents.
1. Discoloration: First Tell Apart Plating from Residue
Q: If a shield can turns yellow after reflow, can it be accepted?
A: First look at where the discoloration is: uniform discoloration over the whole surface is mostly the plating reacting to heat; yellowing only around the solder joints, or at reworked locations, is mostly flux residue.
One pilot-run report compared the same shield can in tin-plated and non-tin-plated versions: of 30 tin-plated units, 2 did not wet and discoloration was 100%; of 90 non-tin-plated units, 0 were defective and discoloration was 0%. An incoming re-inspection in another weekly report also recorded that neither type had obvious warpage, but discoloration was found on the tin-plated type, and R&D had been notified. Whether the discoloration itself affects function, and whether it can be accepted, should be agreed with the customer in the appearance standard.
2. Material: Nickel Silver and Tinplate
Q: Does the shield can material affect discoloration and flatness?
A: Yes. A public article we read mentions that 0.3 mm tinplate (steel sheet tin-plated on both sides) used at first was too hard, and the maker found it difficult to bring flatness within 0.1 mm; switching to nickel silver (a copper-nickel-zinc alloy) made secondary flattening easier, and the material itself takes solder, at a slightly higher price. In design, a pickup area for the nozzle is left near the center of gravity, and corners are reinforced to avoid deformation from stress release.
Unplated nickel silver is not without risk either: in one vibration-test failure analysis, the shield frame was unplated nickel silver, 0.15 mm thick, with pads 0.56 mm wide and only about 0.12 mm from the board edge; most of the solder stayed on the board side, and the judgment was that the solder bonded poorly to the shield frame. Material, pads, and solder volume should be looked at together.
3. Lifting: Besides Solder Volume, Also Check Laser Marking
Q: If one corner of a shield can lifts, is adding solder enough?
A: Not necessarily. In one warpage experiment with 100 units per group: laser marked, no added solder, 1 reflow pass — 3 units lifted, all at the same position; marked with added solder, 1 reflow pass — 0 units. In another case, after enlarging the stencil apertures and adding solder, the customer still reported tilting; compared with another maker’s product, the solder volume was similar, and the main difference was the depth of the laser marking on the top surface — marking is a heating process that deforms thin sheet, and the recommendation was to lower the laser power.
A follow-up marking parameter test confirmed this: the highest-power group showed discoloration in appearance, while the lower-power groups were normal in appearance and defects after 3 reflow passes. For local solder addition, see “How to Set Stencil Thickness and Apertures“.
4. Yellow and White Residue After Rework
Q: What is the yellow and white residue around a reworked shield can?
A: The experimental conclusion in one explanation was: the yellow came from old no-clean flux residue — the board sat for several weeks after production before rework, and the residue turned yellow when heated (rework right after SMT does not cause this); the white came from smoke when the soldering iron touched the solder wire, and wiping with a cloth with very little alcohol leaves white marks, while enough alcohol wipes it off. When replying to the customer, reproducing the phenomenon with old-versus-new board comparisons and smoke and alcohol-amount comparisons is more convincing than a verbal explanation. For whether flux residue should be cleaned, see “Does Flux Residue Need Cleaning“.
5. Manual Rework of a Lifted Metal Lid
Q: How do you save a shield can that has already lifted?
A: One rework procedure is: heat the motherboard on a 170 °C preheating plate → blow around the perimeter with a hot-air gun at 380 °C for about 20 seconds, letting the solder melt, then press flat → add flux around all sides → again 380 °C for about 25 seconds → visual inspection, and redo if it is still lifted. The key points are to preheat first and not to use a high airflow, to avoid blowing nearby small components out of place.
Limitations of This Article
The discoloration ratios and lifting figures come from the pilot run and experiments of a single product, with small sample sizes.
Whether discoloration can be accepted depends on the appearance standard you agree with your customer.
Company names, customer names, and part numbers have been removed from the internal documents.
Frequently Asked Questions
Q: Will a tin-plated shield can always discolor?
A: Not necessarily; it depends on plating thickness, the number of reflow passes, and the peak temperature. In the case in this article, discoloration was 100% under that product’s conditions; different conditions need revalidation.
Q: Should marking simply be dropped?
A: Marking is usually an identification the customer requires; you can first try lowering the power or marking only the outline, then validate warpage and appearance with multiple reflow passes.