Substrate Quality Problems in PCB Laminate Production

Dimension changes on a finished board usually begin long before the finished board exists. They start in the substrate, in the way it was cut, stored, pressed and handled, and they appear later as misregistration, warpage or a stackup that does not match the drawing. The six problems below are the ones that appear most often in production, together with the measures that control them.

Fiber Direction and Dimensional Change

A glass fiber laminate is woven, so it has a warp direction and a fill direction, and it does not expand or contract by the same amount in both. If the orientation is not controlled, the same artwork produces boards with different dimensions depending on which way the material was cut. The remedy is to establish the direction, compensate the artwork with the appropriate shrinkage factor, and then cut the panels so that the fiber direction is consistent. Where the material carries a printed orientation mark from the manufacturer, that mark should be used rather than assumed. Getting this right at the start prevents a class of error that is otherwise very difficult to trace, and the wider consequences are described in this article on PCB dimensional stability.

Stress Release After Copper Is Etched

Copper on the surface and copper in the inner layers constrain the laminate. When a large area of copper is etched away, the constraint is removed locally, and the panel relieves the internal stress by changing shape. The effect is largest where the distribution of copper is uneven, because one region is balanced and another is not. The layout measures are to distribute the circuitry as evenly as the function allows, and at minimum to leave a transition region where the copper density changes so that the stress does not concentrate. Providing copper in the empty areas also balances the etching process, which is why flood copper serves both purposes, and the options are discussed in this article on copper flooding, mesh or solid.

Brushing Pressure

The brushing step that prepares the surface can itself deform the panel. Excessive pressure creates tensile stress in the surface layer, and the board changes dimension as a result. The control is procedural: a trial brush run should be used to establish the parameters that give a clean surface with the minimum pressure, and those parameters should then be fixed. Adjusting the pressure per panel, or increasing it to solve an unrelated surface problem, reintroduces the deformation.

Incomplete Resin Cure

If the resin is not fully cured, it continues to move, and the dimensions change with it, particularly during drilling when the tool loads the material. The remedy is a bake, and the useful place for it is immediately before drilling: a cycle of about four hours at 120 degrees Celsius ensures that the resin is cured before the holes are formed. Baking after drilling does not recover the registration, because the material has already moved under the tool.

Moisture Absorption in Storage

Thin cores and prepreg sheets absorb moisture, and a laminate that has absorbed water does not hold its dimensions when it is heated during pressing. The cause is usually storage conditions rather than the material itself. The control is to oxidize the inner layers as required, bake them to drive off the absorbed moisture, and then keep the prepared panels in a vacuum drying cabinet until they are used. A panel that is left exposed between preparation and pressing will reabsorb moisture, so the interval matters as much as the storage method.

Excessive Resin Flow During Lamination

During pressing, the prepreg resin flows to fill the inner-layer topography. If the flow is excessive, the glass cloth is distorted by the movement, and the panel changes dimension as a result. The control is a designed press trial: the parameters are adjusted until the flow is correct, and the prepreg is then chosen for its resin flow characteristics so that the selected material matches the required fill. Excessive flow also reduces the dielectric thickness between layers, which affects both the voltage withstand and any impedance target, so the same trial protects two requirements at once.

Verifying Substrate Quality

These problems can be managed rather than simply suffered, provided the process is measured. Incoming material should be checked for the manufacturer”s orientation marking and for its dimensional behaviour, and the results recorded against the material lot so that a change of supplier can be distinguished from a change of process. Where a product is being transferred to a new fabricator, the same checks should be repeated, because a board that was dimensionally stable on one line may not be on another. The qualification of a stackup before volume production, described in this article on multilayer PCB prototype requirements, is the natural place to do that verification.

Warpage and How It Is Measured

Some of the effects described above appear as misregistration, and others appear as warpage. The two have different consequences and are measured differently. Misregistration shows up as a layer that does not align with its neighbours, and it is detected by the registration coupon on the panel. Warpage shows up as a departure from flatness, and it is measured by placing the panel on a flat surface and recording the maximum gap, or by measuring the bow and twist as a percentage of the diagonal. The acceptable limits depend on the product class, because a board that must be placed and reflowed on a flat surface tolerates less warpage than one that is hand soldered, and a board that will be assembled on both sides tolerates less still. Baking before drilling helps both effects, because it removes the moisture that drives movement during processing, and the resin that is fully cured is less likely to continue moving when the panel is heated again during assembly.

Where the Responsibility Sits

Some of the causes lie with the material supplier, some with the fabricator and some with the layout designer, and separating them is what makes a recurring problem solvable. Material that arrives with an inconsistent orientation or with a thickness outside specification is the supplier”s responsibility, and it should be detected at goods receipt. Parameters such as brushing pressure, bake schedule, press cycle and prepreg selection belong to the fabricator, and they should be documented as part of the process rather than adjusted by feel. The distribution of copper and the arrangement of the stackup belong to the designer, and they are the part that can be fixed before the first panel is ever pressed. When a dimensional problem appears, identifying which of the three it came from is the first step, and the record of incoming material, process parameters and artwork version makes that identification possible. Without those records, the investigation tends to repeat itself with each new batch.

FAQ

Why does fiber direction matter if the board is symmetric? Because the weave is not symmetric. Warp and fill directions respond differently to heat and moisture, so the orientation of the cut determines how the panel moves.

Why bake before drilling rather than after? Because an incompletely cured resin continues to move under the drill. Baking beforehand ensures the material is stable while the holes are formed.

What controls excessive resin flow? A designed press trial to establish the correct parameters, combined with a prepreg selected for its flow characteristics and its ability to fill the inner-layer pattern.