Re: Types of PCB
Some of them are quite complex. Most are controlled impedance because you are trying to meet EIA or MILSPEC qualification standards. The space on the boards is very limited, and the board must be designed such that it doesn't introduce skew, jitter, false trigger, etc. etc.
In addition to requiring execellent signal integrity, you are faced with high density fanout problems such as 433 ball, 1mm pitch BGA sockets. More and more tester interface boards now require micro vias, blind vias, and filled vias.
Many of the boards today use low Er, high Tg board materials to reduce loss between the tester and the device.
Various methods are used to mount the boards to the testers. Some are held in place by vacuum, so all pads and vias have to be filled, blind, or blank in the vacuum area. Some boards have docking plates to guide the automatic handling hardware. Those boards can't have any parts on the top of the board. Much of the bottom of the board is unusable because of the contact pads that mate to the tester pogo pins.
In short - the boards have to be designed to be as electronically invisible as possible between the device and the tester, and the mechanical resistrictions are numerous.