Packaging Materials as a Source of Contamination
A board that has been cleaned can be contaminated again by the material it is packed in. The contamination is invisible, it appears as an adhesion failure or an intermittent contact, and it is found long after the cause has been forgotten.
Silicone and Its Migration
Silicone is present in release papers, in some foams, in lubricants and in hand creams. A very small quantity migrates across a surface and it lowers the surface energy enough to prevent a coating from bonding.
Once a work surface or a fixture is contaminated, every board that touches it is affected. Our surface preparation notes describe the consequence.
Outgassing From Foam and Plastics
Flexible foam releases plasticiser and other volatiles over time, and a closed container concentrates them against the board. The deposit is thin and it can be conductive or insulating depending on the chemistry.
The material should be qualified for the application rather than bought for the price. Our component selection notes describe the same principle.
Bags, Tape and Labels
Adhesive from a label and the plasticiser in a bag can both transfer to a board. A label placed on a coated area can lift the coating when it is removed.
The placement of labels is a design decision. Our marking notes describe the areas that are usually free.
Work Surfaces and Gloves
A work surface that has been wiped with a silicone containing polish contaminates everything on it. Gloves carry the same risk when they are powdered or lubricated.
The cleaning materials should be chosen for the process. Our floor control notes describe the arrangement.
Detecting the Problem
The detection is usually a dyne test or a contact angle measurement on a surface that should be clean, or an adhesion test on a coating that failed. Both point at the last material the board touched.
The trace should start from the failure and work back. Our coating inspection notes describe the tests.
Rules That Prevent It
Packaging materials are qualified for use near bare assemblies, silicone is excluded from the area, and handling is restricted to the surfaces that are not coated or soldered.
The rules are cheap and they are only effective when they are written. Our cleanliness notes describe the measurement that confirms them.
Verification
The verification is a qualified packaging material list, a dyne test or contact angle on a sample after storage, and an adhesion test on a coated board that has been through the packaging cycle.
Our quality notes describe how the records are kept.
Process Control and Verification
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.
The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. 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.
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
Does a sealed bag protect against this? It concentrates the volatiles rather than releasing them, which is why the material qualification matters.
Can a contaminated surface be cleaned? Sometimes, and silicone is the hardest to remove because it re contaminates the cleaning materials.
What does gopcb provide regarding packaging? We provide a qualified packaging material list, silicone excluded from the assembly area, labels placed where removing them cannot lift a coating, and a dyne or contact angle test plus an adhesion test on a board that has been through the packaging cycle.
