Multi Layer Printed Circuit Board Introduction and Manufacturing process presentation will help you to understand how the miniaturization is possible in electronics and also the Manufacturing of Multi Layer Printed Circuit Board.

2. CONTENTS
What is multilayer board?
Why to use this technology?
How the multilayer boards are
constructed?

3. 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

4. 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.

5. 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

6. 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.

7. Specifications and Standards
1. MIL-STD275 and MIL-P
55110=Military Applications.
2. SCL7503= American Army
Electronics Commands.

8. Equipment
As 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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.

18. Discussion? Questions?