The constant and rapid growth in the field of electronics has led to the advancement in PCB manufacturing processes, increase in the complexity of board design including more internal layers, high density PCB, thinner layers, tight routing schemes and densely populated components on board. These factors are the bases on which the multilayer PCB is designed and fabricated. In this article we shall focus on one of the multilayer PCB manufacturing process called the "Lamination or fusion".
What is PCB layer Fusion..?
As the name implies, PCB fusion means to fuse the multiple layers of PCB with each other. As compared to traditional fusion process the latest fusion process includes the advantages of lower cost of manufacturing, easy operation and process efficiency is high. In multilayer PCB, each conductive and insulation layers are arranged alternately and are bonded together by lamination material. The lamination material is essentially a glue that holds multiple layers together. The combination of fiber glass substrate base material that is the insulator and the epoxy resin is called the pre-preg. The pre-preg is laminated with multiple copper foils and glue / adhesives in between as per fusion technology process.
Rivet Vs Fusion
The traditional rivet technology of fusing / laminating the PCB layers was expensive because of expensive rivets used. Also the stencil used in riveting was prone to damage. Also rivet shape deformation and misplacement on the circuit board made the rivet lamination less desirable as compared to fusion lamination. This riveting method does not use registration pins to hold the inner layers accurately. The pre-preg and inner layers are stacked in book style and secured by rivets. This stack is then depressed with bare metal plates under high temperature and pressure.
While in fusion, the pre-preg epoxy resin is melted under specified temperature and pressure so as to convert epoxy resin from phase B to phase C and internal multiple layers are laminated/glued by adhesives. The resin flow in rectangular shaped fusion joint is more adequate as compared to circular fusion joint. The resin flow must be enough to remove any air bubbles in the PCB layer stack-up. However the resin flow must not be too much such that it will stick the stack with "release film" too much that make it difficult to peel off the release film at the end of fusion process. The properties of fusion depends on the properties of epoxy resin used. The entire lamination process quality is determined by fusion properties and quality. The fusion method is discussed in more detail below.
Factors Influencing the Fusion Process:
1- Fusion Point: The fusion point or joint is the most important factor to make effective fusion process. The fusion point shape can be circle, oval or square. The size of the fusion point should not be too small such that the fusion welding is not solid enough nor the fusion point be too large so that it leaves the white spot due to image penetration, delaminating or lose connection between layers.
2- Position Accuracy: The extraordinary stable and reliable positioning system be adopted so that the internal layers are aligned accurately with each other and reference position.
3- Flatness of lamination press: This is vital that the surface area of lamination press is flat, smooth and hard enough on high temperatures so that homogenous distribution of pressure/force is ensured and controlled vertical movement is balance. The resistive heating element inside the platen hot plate of lamination press is embedded and surrounding is filled with insulator to trap heat and avoid heat loss. The temperature sensor is installed to control the temperature of lamination press. Non-uniform force applied will cause deformation and misplacement of PCB layers.
4- Temperature Settings: The temperature settings are made at 285OC ideally for direct fusion without bond film, but for the FEP bond film to fusion bond the PTFE substrate, the temperature capability of lamination press is 274OC and for CTFE bond film it is 218OC. This is important to abstain from burning the layers, white spotting, de-lamination and de-soldering.
5- Pressure: Normally a laminate size 18" x 24" and CTFE or FEP bonding film will require 100-200 psi pressure. While for direct fusion with thermoset pre-preg 300-500 psi is required. However for sequential fusion for laminates with blind or buried vias for sophisticated and thinner pre-pregs like 2116 PCB design, the pressure can be 100 psi. For thin 0.5oz copper foils, "wrinkle" builds up. This is the pressure due to the glue amount is comparatively large as for copper foils for sophisticated PCB designs.
6- Time: With inadequate fusion time, there is the possibility of uneven and cracked fusion expansion area. Mostly 15 seconds fusion time is adequate.
Main Components of Fusion Process:
The main components used in the fusion lamination process are
1- Lamination Press:
The lamination press is the hydraulic press that applies high pressure under high temperature to press platens. The platens are heated electrically by heating element or oil or steam. Another type is pneumatic or vacuum press, its platens are surrounded by vacuum chamber to remove oxygen / air. The transfer press is the cooling system where water flows through the platens to cool down the heated laminate and cure it. This controlled cooling system (unlike the ambient temperature cooling where cured laminated is left open under open air room/ambient temperature to let the stack cool ) will enhance the quality of laminate for greater throughput.
2- Press Platens:
The press platens gives high amount of heat to carrier press plates. The press platens are hollow inside and are heavy metal. They have pipe running inside and oil, steam or heating resistive element are inside the pipe that generates high temperature. These platens need to remain flat and hard at high temperature because they transfer high amount of heat to carrier plates.
3- Carrier Plates:
These are also know as mirror plates, tooling plate or caul plate. These are ultra hard plates made of AISI630 steel alloy, they are extremely flat and there should be no dent or scratch on the surface of these plates. This can be ensured by regular polishing the surface. It must withstand high chemical reagents during etching. It holds the brushes for tooling pin.
4- Separator Plate:
The separator plate will separate the bond packages / stack panels upon top of each other. It provides the smooth surface to copper foils and it evenly distributes the pressure and temperature differentials (caused by non-homogenous copper distribution / thickness across panel) on the entire panel
5- Release Film:
The purpose of resin film is to prevent/contain the resin flow from sticking the stack together. It also reduces the wrinkles made in the fine/thinner copper foils of less than 0.5oz. The release film enables the copper foil to "slip" rather than wrinkle. The release film can be used between aluminum separator and tooling plate due to differential in Coefficient of Thermal Expansion (CTE) of both.
The purpose of the padding is to compensate for the unparallel, warped and bowed press platen. It also balances the difference in thicknesses in carrier and separator plates. For temperatures as low as 232OC the several lies of thick "kraft" paper is used as paddling. For medium scale temperature the silicone rubber can be used until the temperature is below the melting point of silicone rubber. For high temperatures, woven stainless steel or graphite pad can be used.
7- Kraft Paper:
The kraft paper is used to evenly distribute the heat in the stack so that the difference is as small as possible between the center and sides of the stack