Laminate Selection for High Temperature Boards
Laminate selection is usually settled by price and availability until a product fails a thermal test, and then it becomes the most discussed decision on the board. The properties that matter are the ones that describe how the material behaves when it is hot, wet and mechanically loaded at the same time.
Those properties are measured in ways that are not directly comparable, so a comparison table of two materials is only useful when the test methods are the same. Understanding what each figure describes prevents a supplier’s claim from becoming a design assumption.
Glass Transition Temperature
The glass transition temperature is the point at which the resin changes from a rigid glassy state to a softer one, and above it the material expands faster with every additional degree. It is not a melting point, and a board does not fail immediately when it is exceeded.
What changes above the transition is the coefficient of thermal expansion and the stiffness, so the mechanical and dimensional behaviour of the board deteriorates. Reflow is normally above the transition of a standard material, which is why the number matters even though the exposure is brief.
Decomposition Temperature
The decomposition temperature describes the point at which the resin begins to break down chemically, releasing volatiles and losing mass. It is the property that matters for a board with no thermal vias or for a thick assembly that spends a long time at temperature.
A high glass transition material does not automatically have a high decomposition temperature, and the two are often confused. For a lead free process with a long profile, decomposition is the more relevant figure.
Thermal Expansion and X Y Movement
The in plane expansion of the laminate determines how far a pad moves relative to a component during a thermal cycle. A low expansion material reduces the shear on every joint and is therefore chosen for large packages and for products that cycle in service.
The expansion matters most where a component is large and the joints are small, because the displacement accumulates across the body. This is the mechanism behind the pad cratering that appears on large area arrays after thermal cycling.
Z Axis Expansion and Barrel Stress
The through thickness expansion puts the plating of a via under tension as the board heats, because the copper expands less than the resin. A high z axis expansion coupled with a thick board is the combination that cracks barrels in thermal cycling.
The figure to compare is the expansion over the temperature range the product will actually see, not the value quoted at a single temperature. A material with a low expansion below the glass transition and a high one above it behaves very differently depending on the peak it reaches.
Moisture Absorption
All laminates absorb moisture, and the amount they absorb affects both the dielectric behaviour at high frequency and the risk of delamination at reflow. A material with a high absorption needs a tighter bake and floor life control.
Humidity also changes the glass transition behaviour, since absorbed water acts as a plasticiser and lowers the effective transition temperature. A board that is wet behaves as if the material were a lower grade.
CAF Resistance and Reliability
Conductive anodic filament growth is a failure in which copper migrates along the glass fibre bundles between two nearby holes under bias and humidity. It is a slow failure that appears in the field rather than at test, and it is the reason CAF resistance is specified for high reliability products.
Resistance depends on the resin chemistry, the glass style and the hole spacing, so it is a system property rather than a single material figure. Where the product has many closely spaced vias, the test should be run on a coupon built with the actual stack up.
Halogen Free and Flammability
Halogen free materials remove a family of flame retardants from the resin, which changes the thermal and mechanical performance as well as the environmental profile. The trade is usually a lower decomposition temperature or a higher moisture absorption, so the change should be qualified rather than assumed.
Flammability ratings apply to the finished laminate and are part of the acceptance requirement for most products. A material substitution that keeps the rating but changes the chemistry still needs a re-qualification of the process.
Dielectric Properties at Frequency
At high frequency the dielectric constant and the loss tangent determine the loss of a transmission line and the impedance of a trace. Both vary with frequency and with moisture, so a value quoted at one megahertz does not describe behaviour at five gigahertz.
Where a controlled impedance is required, the material should be chosen with the frequency in mind and the stack up verified with a coupon. The issue is discussed in the context of impedance analysis, where the laminate is one of several contributors.
Cost and Availability
A high performance laminate costs several times a standard one and may have a long lead time, so specifying it everywhere is rarely justified. The usual approach is to use the high grade material only in the layers that need it, which requires a fabricator willing to build a hybrid stack.
Availability matters as much as cost, because a material that is available in one thickness today may not be next year. A design that depends on a single supplier’s specialty laminate should have a documented alternative.
Specifying the Material
The stack up drawing should state the material, the glass transition temperature, the thickness and the copper weight for each layer, and it should reference the tests that support the thermal claims. Vague descriptions such as high temperature grade leave the choice to the fabricator.
The specification should also state the operating environment, since the same board in a sealed enclosure and in a condensing cabinet has two different requirements. This belongs with the rest of the stack up data described in board thickness and fabrication documentation.
Process Control and Verification
On a design of this kind, moisture absorption is the item that decides how the rest of the board is arranged. 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.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
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.
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 temperature always better? No. It usually brings higher cost and a different moisture behaviour, and it should be chosen from the requirement rather than by default.
What causes a via barrel to crack in cycling? The mismatch between the z axis expansion of the resin and the copper, combined with a thick board and a high peak temperature.
Can a laminate be substituted without a test? Only where the properties are equivalent and the process has been qualified. The chemistry change is what makes the substitution risky rather than the headline figure.
Does moisture really affect the material? Yes. Absorbed water lowers the effective transition temperature and raises the risk of delamination during reflow.
