CONSTRUCTION METHODS

The different layers that form the finished multilayer board are laminated together using either the foil lamination method or the cap sheet lamination method. The latter, can be built using single-sided outer cores or using double-sided outer cores.

1) Foil Lamination **
This method is the most cost effective of the 3 alternatives. To build a 4-layer board, 1 sheet of thin-core laminate, 4 pieces of prepreg and 2 pieces of copper foil are needed.

More technical know-how is needed with foil lamination, in order to control the finished thickness, due to the extra prepreg component. The handling of the super thin copper foils is also critical. A distinct advantage of this method is the reduced chance of warpage. Another advantage is that it allows manufacturers without a registration system, to fabricate 4 layer boards.

2) Single-Sided Cap Sheet Lamination
This method is the most expensive of the 3 alternatives. To build a 4-layer board with this method, 3 sheets of thin-core laminate (2 of them single-sided) and 4 pieces of prepreg are needed.

This is the easiest method of constructing a 4-layer board with the major disadvantage being the higher laminate costs. As per foil lamination, an advantage is that 4-layer boards can be made without a registration system.

3) Double-Sided Cap Sheet Lamination
This method is more expensive than foil lamination but cheaper than single-sided cap sheet lamination. To build a 4-layer board, 2 sheets of double-sided thin-core laminate and 2 pieces of prepreg are needed.

Preparing the inner layers with this method requires more labour and photo-resist. As shown in the above drawings, 2 pieces of laminate have to be processed through imaging instead of 1 piece with the other 2 methods, and the outer layers have to be protected from the etch solution. Another disadvantage is the need of a registration system.

REGISTRATION SYSTEMS

Although all PCB manufacturers have the same basic need for good layer to layer registration in multilayer boards, methods of achieving that end vary widely throughout the industry. Many manufacturers use fixed pins, varying in number from 3 to 20 or more. In some cases pinless lamination methods are used.

1) Pinless Lamination
Lamination without pins is a method widely used around the world but generally limited to 4-layer boards. Pinless lamination is used by laminate suppliers in mass lamination production of laminate with built-in inner layers.

Basically this method is based around 2 targets located at each end of the panel. These targets are incorporated on the 2 inner layer films so that they are imaged and etched as part of the inner layer circuitry.

After lamination the outer copper on both sides is surface routed until the inner layer targets are exposed.

These targets are then drilled through and the holes used for drilling location.

A modified pinless lamination method can also be used to fabricate 6-layer boards. Drilling location holes are obtained in the same manner described previously. However, the major difference is registering the 2 inner layer cores which is achieved by riveting both cores using holes which are drilled after etching.

2) Pin Lamination (general)
There are almost as many registration systems as there are PCB manufacturers. Some of these systems were developed by manufacturers for use internally and a good many others by specialist third party equipment manufacturers.

The basics of a system is its ability to aid the registration of innerlayer films to innerlayer cores; innerlayer cores to other innerlayer cores; and finally holes to pressed multilayer panels.

Some of these systems utilise a quantity of registration holes on one or more edges of the panel. A problem which plagues most of these systems is the unpredictability and the unevenness of the dimensional shift inherent to multilayer pressing.

The shift shown in the above diagram will vary from load to load thereby requiring continuous adjustments to be made when drilling, hence the requirement of X-Ray equipment. For boards with more than 4-layers, a best fit situation where no hole breakout occurs can be difficult to find.

When too many points on a board are fixed in place by registration pins, a buildup of internal stresses occurs in the board. These stresses can lead to dimensional instability, warpage and misregistration especially from innerlayer core to innerlayer core.

3) Pin Lamination (4-slot system) **
There is a system which is recognised the world over as the best. It is called the ã4-slot centre zeroing system by Multiline Technology. An indication of its success was IBM changing over from their own system to the 4-slot system in all their manufacturing plants, resulting in a massive decrease in rejects.

The 4-slot system is based on centre line registration with four pins. The system does not pretend to stop movement but instead works on controlling it in a predictable way.

Movement is allowed from the centre out, with any shift being equally shared in both directions thereby preventing the buildup of internal stresses.

The 4 slots are carried right through the multilayer manufacturing process, from film registration, innerlayer imaging, lamination and drilling.

LAMINATION EQUIPMENT

Multilayer manufacturers have a choice of three types of lamination equipment: hydraulic press, vacuum assisted hydraulic press and vacuum autoclave.

1) Hydraulic Press
Conventional lamination is done in a hydraulic press with heated platens which provide uniformly distributed heat across the entire multilayer package. Most local PCB manufacturers use this type of press. The cycle involves the loading of stacks for pressing, heating followed by cooling.

The hydraulic lamination system is a proven one, albeit with some limitations, especially on denser multilayers. When a multilayer is pressed, there is a significant amount of resin that flows out of the package, the result of the applied pressure. This not only causes distortions in the innerlayers, but can also cause a ãresin starvedä areas. This condition gives a higher dielectric constant and can result in delamination under heat stress. Air bubbles could also be trapped under the innerlayers, causing blisters and possible drilling and plating voids.

2) Vacuum Assisted Hydraulic Press **
In recent years, with the increasing demand for more sophisticated MLBs with finer lines, smaller holes and more layers, considerable attention has been given to alternative lamination equipment. The introduction of vacuum lamination presses proved to be a major step forward. In this approach, the platens are enclosed in a vacuum tight chamber, permitting lamination to take place in a vacuum environment. The cycle is somewhat similar to conventional hydraulic pressing with the one major difference of vacuum drawdown prior to the application of pressure.

With the aid of vacuum, a lot less pressure is required, resulting in less distortions and virtually no air entrapment. Another plus is that very little resin flows from the multilayer. The finished result is a board with lower dielectric constant, preferred for impedance controlled boards which are becoming very dominant in todayâs market.

3) Vacuum Autoclave
Different to the other two systems, autoclave lamination consists of a vacuum bagged fixture within a heated, pressurised vessel. Pressure is applied by means of gas compression. The gas used is either nitrogen or carbon dioxide. The heat is then applied and transferred with the gas acting as the transfer medium.

The first autoclave became available in the mid 80s. The lamination cycle starts with vacuum pull down, followed by pressurising, heating and eventually cooling.

One of the major advantage of the autoclave lamination system is that all resin remains within the multilayer due to the inward pressure. However, a potential problem is that since there is almost no resin flow, it is possible to get spot resin voids.

Before electroless copper deposition, the drilled holes on multilayers have to be desmeared. The main purpose of this treatment is to remove any epoxy smeared onto the edge of the inner layer copper pads whilst drilling. The smear is the result of a hot drill melting the epoxy resin and spreading it across the hole wall during its up and down motion. The epoxy smear causes poor and in some cases non electrical continuity to the innerlayers as well as poor adhesion of the plated copper to the hole wall.

In desmear, the chemical attack is designed to remove only the surface smear of resin from the hole wall, without attacking the wall structure itself.

For most needs desmearing is sufficient whilst for some military applications it is necessary to etchback 75mm for what is referred to as 3 point contact.

Etchback is a stronger chemical attack than desmearing. It is designed to remove not only the smeared resin, but also additional resin from the wall structure and the remaining protruding glass fibres.

There are various methods of desmearing and etchback available to multilayer manufacturers.

1) Sulphuric Acid
Concentrated sulphuric acid works on the principle of promoting the dissolution of the epoxy smear. Advantages are its low running cost and ease of operation. Disadvantages are its high working speed making it virtually impossible to adopt to automatic lines. its short working life due to the hygroscopic nature of the acid and its poor wettability of the solution making it unsuitable for small holes.

2) Chromic Acid
Chromic acid works on the principle of oxidising the epoxy smear. Advantages are its predictability, its wide operating window and its long solution life. Disadvantages are its high toxicity and costly waste treatment.

3) Alkaline Permanganate **
Permanganate works on the principle of oxidising the epoxy smear. Advantages are its predictability, its wide operating window and its ability to handle most resin systems. Permanganate has virtually no disadvantage.

4) Plasma
In the plasma system, the boards are exposed to a mixture of gases in a partial vacuum and a high frequency field. Advantages are its ease of operation being a non-wet method and its ability to handle all resin systems. Disadvantages are its high entry cost for equipment and a residue ash synonymous with plasma etching which needs to be removed off line.