Steps to increase the reliability

Favourable aspect ratio

picture 1

Aspect Ratio (AR)

1 Via, AR 1:6

2 Via, AR 1:8

3 Via, AR 1:9

4 Blind Via, AR 1:1

5 Blind Via, AR 1.5:1

In general you can say: the lower
the aspect ratio, the better the
reliability of the board.
Military applications are exemplary,
an aspect ratio of max. 1:6 vor vias
and 1.5:1 for blind vias is
recommended here.

For all the other kind of boards we recommend an aspect ratio of max. 1:9 for vias and 1:1 for blind vias.

Use of non functional pads

picture 2

photo 3

photo 4

In contrast to this the Hans Brockstedt GmbH strongly recommends placing a pad in each inner layer to increase the reliability of the board (even in the clearance areas of GND/VCC) to achieve extra stabilisation of the plated barrels. This step reduces the risk of barrel cracks. (See picture 2 and photo 4)

The advantages of adhesiveless laminates combined with window technology

picture 5

photo 6

Construction with adhesiveless laminates combined with window technology
Picture 5 and photo 6 show a construction with adhesiveless flexible laminates and the use of window technology. The construction with adhesiveless laminates is not enough to increase the reliability of a board. In addition the window technology should be used. This means that the cover foil is only partial laminated on the flexible area. The foil should project at least 1 mm into the rigid part of the PCB.
The window technology allows constructions, which are free of acrylic adhesive layers in the rigid part of the multilayer and avoids by this the problem of the high CTE of the acrylic adhesive in the z-axis.

picture 7

photo 8

Construction with acrylic adhesive laminates
As shown in picture 7 and photo 8 this construction contains eight 25 µm acrylic adhesive layers (shown in green). The adhesive layers are also inside the rigid part where you usually find the through-hole plating as well. Due to the high CTE of the acrylic adhesive in the z-axis the construction of the circuit is poor.