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

On a design of this kind, glass transition is the item that decides how the rest of the board is arranged. 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.

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.

Process Control and Verification

On a design of this kind, glass transition is the item that decides how the rest of the board is arranged. 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.

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.

Process Control and Verification

On a design of this kind, glass transition is the item that decides how the rest of the board is arranged. 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.

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, glass transition is the item that decides how the rest of the board is arranged. 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.

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

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.