Laminate Selection for Lead Free Reflow Temperatures

Lead free reflow runs about thirty degrees higher than the tin lead process it replaced, and the laminate has to survive that. The properties that matter are the glass transition temperature, the decomposition temperature and the expansion below and above the transition.

Glass Transition Temperature

Below the transition the laminate is stiff and its expansion is low. Above it the material softens, the expansion rises sharply and the modulus falls.

A board whose transition is below the reflow peak spends part of the process in the softened state, which changes how it behaves under the weight of the components. Our warpage notes describe the consequence.

Decomposition Temperature

The decomposition temperature is where the resin begins to break down and release volatiles. It is well above the transition and it limits how many times the board can be reflowed.

Each reflow consumes part of the margin, so a board that needs two passes or a rework should have a higher decomposition temperature. Our rework notes describe the thermal exposure that adds to it.

Expansion Below and Above the Transition

The expansion in the plane of the board is set by the glass fabric and the expansion through the thickness by the resin. Above the transition the through thickness expansion rises steeply.

The expansion through the thickness loads the plated barrels, which is why a high transition material is used on a board with a high aspect ratio. Our aspect ratio notes describe the geometry that makes the load worse.

Moisture and the Reflow

The laminate absorbs moisture, and the absorbed water turns to steam during reflow. The pressure it generates can delaminate the board or produce a blow hole in a via.

The moisture level should be controlled by storage and, where necessary, by a bake before assembly. Our storage notes describe the control that is applied.

Time Above Liquidus

The laminate sees the peak temperature for the time above liquidus, and the damage accumulates with that time rather than with the peak alone.

A profile with a long soak and a long time above liquidus damages the board as well as the components. The time should be the minimum that produces a sound joint. Our profile notes describe how it is measured.

Choosing the Material

The choice follows from the number of reflow passes, the board thickness and the aspect ratio, the rework expectation and the thermal cycling the product will see.

Specifying a higher grade than the process needs costs money, and specifying a lower grade produces a board that delaminates on the second pass. The requirement should be stated explicitly. Our laminate notes describe the properties that are compared.

Verification

The verification is the thermal analysis of the material from the supplier and a test of the finished board through the reflow profile it will see.

Where the board is to be reworked, the test should include the rework exposure rather than only the assembly. Our quality notes describe how the result 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.

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.

Verification and Records

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. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

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. Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it.

FAQ

Is a higher glass transition always better? It is better for the process and it usually costs more and it can be more brittle. The requirement should follow the process and the application.

Does a bake remove all the moisture? It removes the absorbed moisture to a level and it does not remove the moisture that has already reacted with the resin.

What does gopcb provide for laminate selection? We provide material selection against the number of reflow passes, the aspect ratio and the rework expectation, storage and bake control before assembly, a profile with a controlled time above liquidus, and verification of the finished board through the thermal exposure it will see.