Dry Pack Process Control

Dry packing is the process of sealing moisture sensitive material in a barrier bag with desiccant so that the exposure clock stops. The controls are simple, and their failure is invisible until a package cracks at reflow.

The process has three parts: the bag and its contents, the seal and the record. Each can be verified, and each is often assumed.

The Bag and Its Contents

The bag should be sized to the contents, since a large bag is difficult to seal flat and is easily punctured. The material should be placed so that it cannot puncture the bag from inside.

The desiccant quantity follows from the bag volume and the expected exposure, and the indicator card is what shows whether the protection held. A bag without a card provides no evidence.

Seal Quality

The seal is the weak point, and it should be wide enough and continuous. The sealing temperature and pressure have to suit the bag material, and a seal that is too hot becomes brittle.

The seal should be checked on a sample, either by a peel test or by a visual inspection of the width. A partial seal is worse than no seal, because the card may not show the leak.

Humidity Indicator Cards

The card changes colour at defined humidity levels, and the reading should be recorded at the point of opening. A card that shows exposure means the material has been absorbing moisture, whatever the cause.

The card should be read in the specified conditions and within the time stated, since the reading drifts once the bag is opened.

Resealing and Partial Use

A bag that is opened for part of its contents should be resealed with fresh desiccant, and the exposure recorded. The clock starts again when the bag is opened next.

Where the material is used repeatedly, a dry cabinet is more practical than repeated sealing and should be used as described for storage and humidity control.

Records

The record should show the bag identity, the date opened, the card reading and the exposure accumulated. That record is what allows a decision when the floor life is reached.

It should be kept at the station where the material is opened rather than in an office, since the decision is made there. The practice is described for moisture sensitivity levels.

Baking and Recovery

Material that has exceeded its floor life is baked on the schedule for the package, and the bake count recorded. The material is then repacked or used.

The bake schedule and the limits are described with the same practice at moisture bake.

Audit

The process should be audited by checking the seal on a sample, the card readings and the records. An audit that only looks at the records does not test the seal.

The audit findings usually concern the records rather than the material, which is a sign that the physical process is understood and the documentation is not.

Records Retention

The records should be retained for the life of the product, since a moisture related failure appears long after the material was used.

They belong with the process evidence described for manufacturing processes.

Process Control and Verification

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.

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.

FAQ

Is a card enough evidence of protection? It shows whether the bag held, and it does not replace a seal check on the process.

How much desiccant is needed? It follows from the bag volume and the expected exposure, and the supplier’s table should be used.

Can a bag be resealed more than once? It can, with fresh desiccant each time and a new record of the exposure.

What is the most common failure? A seal that is incomplete, which the card may not reveal if the leak is small.