CONTENTSWhat is multilayer board?

Why to use this technology?

How the multilayer boards are constructed?

What is multilayer board?

L1

Vcc

Gnd

1

4

3

2

The sandwich of two or more double sided PCB’s is known as multilayer PCB.In multilayer PCB, layer one and four are used for signal interconnections and layer two and three are used for Vcc and Gnd.L4

Why to use this technology?As man has started accepting the things

which are small in size, multi-functional and easy to use. He started switching to the different technologies such as multilayer board.

Where the SSB and DSB could not meet our requirements, it become necessary to use multilayer board.

How the multilayer board are constructed?

There are many types of multilayer board construction. The different types are distinguished by the method used for making connections from one layer to another.

There are three inter-connecting techniques which are newly used :-

1. Through via

2. Blind via and

3. Buried via

1. Through via= To connect 1st and 6th or top to bottom layer.

2. Blind via= To connect 1st to 3rd or 1st and 2nd layer or surface layers to inner layers.

3. Buried via= To connect 2nd and 3rd or 4th and 5th or inner layers.

Specifications and Standards

1. MIL-STD275 and MIL-P 55110=Military Applications.

2. SCL7503= American Army Electronics Commands.

EquipmentAs most of the of multilayer board fall

into glass epoxy category, we shall discuss the equipments associated with it .

1. The pressure is needed to make laminates with void-free intermediate epoxy layers.

2. The pressure is applied to drive the air bubbles out from the B-stage resin towards the edges of the board.

3. As some air bubbles get trapped in the edges by the time curing starts, a border of perhaps 25mm[2.5cm] on each side should be left for trimming.

4.For this purpose the pressure of 25 tons(25,000Kg) and temperature of 180⁰C is required.

5.The press must be capable of maintaining an even temperature over the laminate area.

6. A clearance of about 150mm[15cm] between the platens is necessary to enable easy insertion and removal of the circuit boards.

7. The press plates are usually of steel or aluminum and are about 6.4mm thick which provides better dimensional stability of the boards.

Lamination Process

Preparations:-

This stage of lamination process concludes, the alignment of all layers such as top layer, prepreg and bottom layer.

For safety, it is important to do a very careful layout for each one of the layer in order to prevent masking of needed holes.

Preheating:-

1. The platens and the press plates are preheated in the equipment to a temperature of 180⁰C.

2. Then the successive layers of laminate and prepregs are pressed carefully into position.

3. Kraft paper used for equalizing pressure, is then placed between the platens.

4. If low-flow B-stage material is used for prepregs, Press can be closed and full pressure can be applied for not less then 45 minutes.5. To prevent warping of the finished board after curing, it is good practice to allow the board to cool in the press under full pressure down to 60⁰C or even lower.6. If high-flow B-stage material is used, the timing of the application of full pressure has to be studied.

Post-Laminating inspection:-

1. After removal from the mould, the laminate is inspected for insulation resistance as design requirement.

2. Radiography method is used to study the alignment of the layers.

3. The board is then trimmed of excess material{25 mm from all sides} and drilled.

4. It is important to use a sharp drill bit and to adjust feed and speed to minimize epoxy smear.

Further Processing

Inspection of the holes:-

1. The boards then need a thorough inspection to verify the absence of epoxy smear in the holes.

2. After the inspection they are given for further treatment to build up the plated through holes as it is similarly done with ordinary DSB’s.

Etch Back:-The epoxy smear is removed and the exposed glass

fibers is prepared for copper deposition is known as etch back.

Etchants used in etch back:-1. It is a standard practice to immerse the board for 10 sec in Sulphuric acid of a concentration exceeding 85%, which removes the epoxy smear.

2. Then the board is dipped in Hydro-fluoric acid for 10 sec, which removes exposed fibers.

3. In between both the steps, the board is rinsed 2 to 3 times with high-pressure deionized water.

4. These operations are hazardous and hence the operator has to be adequately protected.

5. The last stage is to plate the copper through holes of multi-layer boards.

Discussion? Questions?