Glass Transition Temperature and Its Use in PCB

A laminate has a temperature at which its mechanical behaviour changes, and the change is what the glass transition temperature describes. Above it the material expands faster and becomes softer, which is the condition the assembly processes create.

What the Transition Is

Below the transition the resin is glassy and stiff, and above it the resin becomes rubbery and its expansion coefficient rises by a factor. The transition is a range rather than a point, and the figure quoted is a characteristic of that range.

The transition is measured by a thermal method on a sample, and the method affects the figure. Two figures quoted from different methods are not directly comparable. Our laminate properties notes describe how the material is characterised.

Why It Matters for Assembly

An assembly process takes the board above the transition, and in that state the board expands more and resists less. The expansion is what drives the strain in a plated through hole and the movement during lamination.

A material with a higher transition spends less of the process above it, which is the reason a high transition material is chosen for a thick board or for a board that sees many thermal cycles. Our thermal cycling notes describe how the consequence is measured.

The decomposition temperature is where the resin begins to break down chemically, and it is much higher than the transition. It sets the limit for the processes that follow, such as a heavy rework or a hot finish.

The two figures are often quoted together and they answer different questions. A material with a high transition and a low decomposition is a material that resists expansion and not heat. Our board quality notes describe how the process limits are applied.

The choice is made from the process the board will see rather than from the transition alone. A board that is assembled once with a lead free profile and used at ambient does not need the same material as one that is reworked and cycled.

The second consideration is the thickness, because a thick board carries more expansion over the length of a barrel. The two are evaluated together in the stack-up.

The transition affects the lamination, because the press takes the resin above it and the material then flows and shrinks as it cools. A material with a different transition behaves differently in the same press cycle.

That is the reason a material substitution is a lamination change rather than a purchasing one. Our <a href="https://www.gopcba.com/pcb-warpage-control/” title=”warpage”>warpage notes describe the result of an unbalanced press cycle.

The transition is a material property and it is verified on the incoming material or on a coupon. A product measurement is possible with a thermal method and it is rarely done, because the figure describes the material rather than the board.

The useful product measurement is the consequence: the barrel strain, the warpage and the delamination resistance. Those are measured directly.

Acceptance and Its Evidence

On a design of this kind, glass transition is the item that decides how the rest of the board is arranged. A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made. Where the supplier and the user both measure the same property, they should agree on the method before the first delivery.

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 first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.

The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected. Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion.

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. The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold.

A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed. 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.

Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it. A change that is not recorded is a change that cannot be explained when the result moves, which is why the record is part of the process.

Checks Before Release

On a design of this kind, glass transition is the item that decides how the rest of the board is arranged. 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. Consumables have a life measured in cycles, and the replacement point should come from the measurement rather than from a failure.

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. The narrowest feature on the board usually sets the process window for the whole product, so it deserves the closest attention at review.

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record. Where a process is at the edge of its capability, the margin should be bought deliberately rather than discovered during production.

Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first.

Verification and Records

On a design of this kind, glass transition is the item that decides how the rest of the board is arranged. 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. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

The sequence of operations is part of the specification, because a different order produces a different result from the same steps.

Points to Confirm at First Article

On a design of this kind, glass transition is the item that decides how the rest of the board is arranged. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts.

Is a higher transition always better? It resists expansion longer and it usually costs more and processes differently, so the choice follows the requirement.

Is the transition the maximum temperature? No. The decomposition temperature is the limit and the transition is a mechanical change below it.

What does gopcb provide for laminate selection? We provide a transition specified from the process the board will see, the measurement method stated with the figure, the decomposition limit used for the hot processes, the material evaluated with the board thickness, and the lamination qualified when the material changes.