Solder Wick and Desoldering Practice
Solder wick is a braid of fine copper strands that draws molten solder away from a joint by capillary action. It is simple, and it damages more pads than any other hand tool when it is used at the wrong temperature or for too long.
How the Braid and Flux Work
The braid provides the capillary path and the flux reduces the surface oxide so that the solder can enter it. Flux cored braid carries its own, and separate flux is applied where the joint is heavily oxidised.
The braid is a consumable and its width should match the joint. Our joint criteria notes describe the acceptance that follows.
Temperature and Dwell
The iron should be hot enough that the solder melts quickly and cool enough that the flux is not burnt off. The dwell should be short, and the iron should be lifted as soon as the solder has been drawn.
A long dwell is what lifts a pad. Our rework heating notes describe the alternative methods.
Why Pads Lift
The pad is bonded to the laminate by an adhesive strength that falls as temperature rises. Heat applied through a wick is concentrated on a small area, so the pad reaches a higher temperature than the board around it.
Preheating the board reduces the local temperature needed. Our site preparation notes describe the practice.
Through Hole Barrels
Wicking a through hole removes solder from the barrel but it rarely removes all of it, and the remainder has to be cleared before a new lead goes in. The barrel plating is thin and easily damaged by repeated passes.
The barrel should be inspected after clearing. Our hole copper notes describe the plating.
Cleaning the Site Afterwards
The flux from the braid remains on the site and it has to be removed before the new joint is made, or it becomes the source of a void. The cleaning should suit the flux chemistry.
Our voiding notes describe the mechanism.
When Not to Use a Wick
A very fine pad or a fragile laminate is better served by a hot air tool with a controlled temperature. A large ground plane joint is better served by preheating the board rather than by holding the iron on it.
The choice should be made before the work starts. Our quality notes describe the acceptance of the finished site.
Verification
The verification is a site inspected under magnification after wicking, a check for residual solder and flux before the new joint, and a barrel that is intact after clearing.
Our first pass yield notes describe how the outcome is recorded.
Process Control and Verification
Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.
Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.
Checks Before Release
The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.
A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.
Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.
The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.
FAQ
Should the wick be pressed onto the joint? It should be held in contact with light pressure. Pressing harder improves the thermal contact and increases the chance of lifting the pad.
Does a wider braid remove more solder? It removes it faster and it covers more of the site. The width should match the joint rather than be maximised.
What does gopcb provide during desoldering? We provide a braid width matched to the joint, a temperature and dwell that melt the solder without lifting the pad, preheat for large plane connections, a cleaned site before the new joint, and a magnified check of the pad and barrel afterwards.
