Multilayer is quickly becoming the standard.  Low layer count products represent the largest proportion of these types with 4 layer boards outnumbering almost all other layer counts combined.

Eddy Electronics can produce up to 12 layer printed circuits with what is in stock at all times.

Multilayer boards are made by etching thin double sided panels ( cores ) and laminating them together with uncured fibre-glass sheets ( prepreg ).  Alignment is accomplished with a series of tooling holes and pins.  When the laminated panel is drilled and plated through, the vertical connections are made.

Blind or buried vias are inner connections made when the cores are plated through prior to lamination.  This allows multiple connections to be made at a single X-Y coordinate,  the vias being isolated in the Z (vertical) direction.

Internal ground and power planes are useful for efficient distribution of power and the elimination of noise.  Ground loops on audio or RF products can be rendered impossible.  Digital and analog signals can isolated completely with little effort.  Impedance can be controlled in critical areas to improve high speed, high power or low noise applications.  The coolest application of internal metal is as heat sink.  The space program could probably not exist with out it!  Power density of a printed circuit product can be increased enormously.

Obviously, this technology allows vast flexiblity to the designer, and coupled with options in substrate type and copper thickness, a great deal of product capability can made on a single circuit board.

Check with us or ask  specific questions before you get commited to a design.  There are things you need to know!

This is a shot that illustrates a useful design for manufacturability (dfm) tool.  The window in the centre has 8 panes, one for each layer in this 8 layer backplane.  One can tell that each piece of film has its number assigned in a way that prevents any confusion.  The final product is verifiable in the sense that one can tell how the layers were laminated.  The order of lamination has to be specified so that the desired effects of multilayer construction are achieved.

 

You can also see internal tracks running under the surface near the bottom and top of this board.