Vacuum Lamination And Void Control In Multilayer Boards

A multilayer board is made by pressing several layers of copper and prepreg together until the resin flows, fills the gaps around the copper and cures into a solid block. If the resin does not fill a gap, the result is a void, and a void inside a board is a place where the layers are not bonded, where moisture can collect and where the board can separate during soldering. Vacuum lamination is the process that removes the air so the resin has somewhere to flow.

This article explains where voids come from, how a vacuum press removes them, how the press cycle is arranged, and how the result is inspected.

Where Voids Come From

Three mechanisms produce them. The first is trapped air, which is present in the weave of the glass fabric, between the layers and in the holes that were drilled or punched in the prepreg. The second is volatile material released from the resin as it heats, which includes moisture and the by products of the curing reaction. The third is a failure of the resin to flow, which happens when the viscosity is too high, when the pressure is too low or when the flow has to travel too far.

The geometry of the copper sets the difficulty. A heavy copper layer with a thick trace creates a step that the resin has to fill, and a board with a large area of copper surrounded by areas with none has a very uneven flow. The stackup that results is designed with this in mind, and the principles are described under layer stackup from one to eight layers.

How A Vacuum Press Works

The book, which is the stack of boards and separators, is placed in a press and the chamber is evacuated before the pressure is applied. Removing the air before the resin becomes fluid means that the air in the weave and between the layers is drawn out rather than compressed into the middle of the board, and the resin then fills the space that is left.

The sequence is a set of stages: evacuate, apply contact pressure, heat, increase pressure as the resin softens, hold, cool under pressure, release. Each stage has a temperature and a time, and the point at which the pressure is raised is critical. Raising it too early compresses a resin that is still too viscous to flow and traps the air; raising it too late allows the resin to gel before the gaps are filled.

The Press Cycle And Resin Flow

The resin has to reach a viscosity low enough to flow into every gap and high enough not to be squeezed out of the edges of the panel. That window is narrow, and it is defined by the chemistry of the prepreg and by the rate at which the temperature is raised. A slow ramp widens the window and lengthens the cycle, while a fast ramp shortens the cycle and increases the risk of trapping air.

The pressure then has to be high enough to close the stack and low enough not to distort the inner layer patterns. The resin that flows out of the panel, called the flash, is a consequence of the pressure and is measured as an indicator of the flow: too little flash means the resin did not move, and too much means it moved too far and left the edges resin starved.

Material And Stackup Contributions

The prepreg carries most of the responsibility. Its resin content, its flow characteristics and its age all change how it behaves, and a prepreg that has absorbed moisture will release volatiles during the press. The material is stored in a controlled environment and used within its shelf life for that reason, and the volatile content is specified and tested.

The stackup contributes through symmetry and through the copper distribution. An asymmetric stackup flows unevenly and tends to warp, and a layer with a large copper area next to one with almost none creates a pressure difference across the panel that shows up as a thickness variation. The dimensional behaviour that results is described under PCB dimensional stability and expansion, and the electrical consequences of the same structure are described under multilayer PCB advantages at high speed.

Detecting Voids

A void inside a board is not visible from outside, so it is found by destructive or by ultrasonic methods. A microsection through the panel shows the layers and the gaps between them, and it is the reference method: a void appears as a region where the resin is absent and the layers are separated. Sections are taken from a coupon that is built with the production panel rather than from the product itself.

An ultrasonic scan images the whole panel by detecting reflections at the interfaces, and it can be used on production panels without destroying them. The method is sensitive to a bonded area versus an unbonded one, and it produces a map that shows where the delamination is. It is used for a product with a strict requirement and for investigating a complaint, since it is slower than a section and needs a reference sample to interpret.

Symptoms And Causes

Blistering after reflow is the classic symptom of a lamination problem, and it appears as a raised area of mask or of laminate around a copper feature. It indicates that the bond was weak and that the moisture driven off during reflow pushed the layers apart at that point. A delamination that appears as a light line in a section and does not blister has usually existed since the press and is a resin flow problem.

Measling, the appearance of small light spots inside the laminate, is a related condition in which the glass bundles are not fully wetted. It reduces the mechanical strength and the dielectric performance, and it is a sign that the resin content or the press cycle is out of range. Both conditions are process indicators, and the corrective action is in the material or the press settings rather than in the layout.

Verification And Control

The press parameters are recorded for every cycle: the vacuum level, the temperatures, the pressures and the times at each stage. That record, together with the flash measurement and a section from the coupon, is the evidence that the lamination was sound. A single missing record makes a later failure impossible to attribute.

The incoming material is verified as well. The resin content, the flow and the volatile content of the prepreg are tested on receipt, and the results are kept against the batch. When a blistering problem appears, the first question is whether the prepreg batch changed, and the answer is available only if the incoming test was performed.

FAQ

Does vacuum lamination remove all voids? It removes the air that would otherwise be trapped, but it cannot compensate for a resin that does not flow or a stackup that cannot be filled. The process removes one cause of the problem, not all of them.

Why does blistering appear only after soldering? Because the void and the moisture were present but not visible, and the heat of the soldering operation turned the moisture into steam and separated the layers. Without the thermal event the board looks perfect.

Can a board with lamination voids be reworked? No. The layers are already bonded and a void is inside the structure. The remedy is a bake to remove the moisture, or a change of process for the next lot rather than a repair of the finished board.