Solder wire is soft enough to deform under its own weight given time. So are several other soft alloys and some pure metals. For a spool, this turns everything about tension and support into a problem the product cannot help you with.
The core problem
Stiff product contributes to holding its own shape. Soft product does not. That means:
- It takes permanent deformation readily. Press it against anything and it stays pressed — the same mechanism discussed for set in our article on product curl, except here it is much easier to trigger.
- It has no resistance to compression. Layers press into each other under nothing more than their own weight.
- It will cold-flow. Given time under load it moves, slowly, which is one reason long storage is harder on it than on anything else in this series.
Flattening between layers
The specific failure that catches people out. Where the wire sits against the layer beneath, contact pressure is concentrated on a line. Over weeks, the wire flattens slightly along that line. At payoff it comes off with a flat rather than round section, and it feeds badly through anything expecting round.
Mitigations, roughly in order of usefulness:
- Lower winding tension — the main lever
- Smaller package depth — less accumulated pressure
- Larger core diameter — lower curvature at the base of the pack
- Interlayer separation — see the discussion in our interlayer sheets article, though note the handling cost there is real and it does compound here
Nesting
Soft wire can bed down between adjacent wraps, particularly if the traverse leaves gaps or if turns are not laying cleanly side by side. Once one turn has partially settled into its neighbour, pulling either of them out is a fight, and the usual result is deformation of the product.
The prevention is a clean consistent traverse, discussed in general terms in our article on traverse patterns. The stakes are simply higher when the product cannot push back.
Temperature
More important than for almost anything else in this series. Soft metals and low-melting alloys become markedly softer as temperature rises, and the rate of cold flow roughly follows. A spool stored somewhere warm for a summer will have deformed in ways the same spool stored in a cool building would not.
If you are seeing seasonal variation in soft wire problems, this is the first thing to check, and it interacts with the general storage discussion in our piece on long-term wound storage.
Core diameter
Larger core means lower curvature and less deformation of the innermost layers. The trade is packaging efficiency, as always. For soft product the balance usually shifts further towards a bigger core than people expect, because the alternative is scrapping the inner layers.
Some operations accept that the innermost layer is sacrificial and do not wind to zero. That is a legitimate economic decision if you make it deliberately.
Handling
A full spool of soft wire is easily damaged by impacts that would not trouble anything else, because nothing in the pack has the stiffness to resist. Dropping, or resting on the rim, or stacking too high, all press product that cannot recover.
The storage principles in our storage yards article apply with lower limits than they suggest for general product.
What we would tell you honestly
If you spool very soft wire, expect to over-specify the packaging relative to what the weight suggests. The load numbers look trivial and the consequences of getting the approach wrong are disproportionate.
On our hollow board spools the relevant variables are core diameter, surface finish and flange clearance, and we can set all three. What we cannot do is compensate for a spool being stored somewhere hot for two months — that one is on your process.