Rework Site Preparation and Pad Repair

Rework begins with the site, not with the component. A site that still carries the old solder, the old flux and the heat history of the previous assembly will not produce a joint as good as the original, whatever tool is used.

Removing the Old Solder

The old alloy has to be removed completely, because mixing two alloys produces a joint with a melting range and a structure that neither was qualified with. Wick and suction both work and both leave a residue.

The removal should be checked under magnification. Our joint criteria notes describe the acceptance that follows.

Flux and Cleaning

The flux used for removal has to be cleaned off before the new flux is applied, because two chemistries on one site behave unpredictably. The cleaning should be appropriate to the flux.

The residue is often the reason a rework joint voids. Our voiding notes describe the mechanism.

Preheat and Its Purpose

Preheat reduces the temperature the local tool has to supply and it reduces the thermal gradient across the board. A site that is not preheated is heated locally and the gradient is what damages laminate.

The preheat temperature is set below the solder melting point. Our rework heating notes describe the methods.

Laminate Damage

Excessive local heat produces a discoloured or lifted pad and, at the extreme, a blister in the laminate. The damage is cumulative, so the number of rework cycles at one site should be limited.

The limit should be written rather than left to the operator. Our laminate notes describe the behaviour that follows.

Pad Repair

A lifted pad can sometimes be repaired with a bonded replacement or an epoxy and copper repair, and sometimes the repair is not acceptable for the product. The decision should be recorded.

The repaired site has to survive the same thermal exposure as the original. Our thermal cycling notes describe the test that confirms it.

Jumper Wires and Their Control

A jumper is a route, and it should be drawn and recorded rather than improvised. An unrecorded jumper is a defect that appears in the field as an unexplained continuity.

The record belongs with the assembly. Our design release notes describe the change control that covers it.

Verification

The verification is a site inspected under magnification after removal, a preheat temperature recorded, a limit on the number of cycles at one site, and a record of any pad repair or jumper with the person who performed it.

Our quality notes describe how the records are kept.

Additional Considerations for This Build

Practical attention to site preparation pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating site preparation explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, laminate damage is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. 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.

Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

Process Control and Verification

On a design of this kind, laminate damage is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. 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.

Process Control and Verification

On a design of this kind, laminate damage is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. 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.

Process Control and Verification

On a design of this kind, laminate damage is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. 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.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Can the old solder be left in place? It can where the same alloy is used and the joint is reflowed fully, and it should not be where the alloy differs or where the joint is only partially melted.

Is a repaired pad as good as the original? It can be mechanically adequate, and it should be treated as a deviation that is recorded rather than as an equivalent.

What does gopcb provide during rework? We provide a site cleaned of the old alloy and flux before new material is applied, a preheat temperature recorded for the assembly, a written limit on cycles at one site, and a record of every pad repair or jumper with the operator and the method.