Multi-Layer Constructions

Manufacturing of PTH multi-layers starts with thin core laminate that is imaged, developed and etched, surface treated for bonding and then sandwiched with prepreg and copper foil in a special press using pressure and heat to stick the materials together.  After pressing, the multi-layer panels then follow the normal double layer process with the addition of a special cleaning treatment after drill.

Despite some popular opinion, blind or buried vias complicate the multi-layer process as it involves extra drill, plating & press cycles as well as hole/surface cleaning problems.  

Nominal thickness of prepregs carried in stock:

  • 7628 – 6.5thou or 0.165mm
  • 1080 – 2.5thou or 0.0635mm

When 2 pieces are pressed together, the overall spacing (dielectric) is generally thinner than twice the nominal thickness above due to the “meshing in” that occurs in the press cycle.  Prepreg is glass cloth with resin and, like any cloth, has weaves that leave “holes”.  This can be made worse by the pattern of the copper either side (planes or signal layers).  Prepreg is a perishable material and thickness will vary due to atmospheric & storage conditions as well as age of material.  The fresher the prepreg, the “wetter” (more resin = more prepreg flow = thinner build) or the older the prepreg, the “dryer” (less resin = less prepreg flow = fatter build). 

Tolerances on multi-layer thickness is typically +/-0.2mm, remembering that board thickness is measured over the fibreglass, not the copper.

1. Standard 4 layer PTH, 1.6mm thick, 1 oz copper all layers

  • 0.5oz foil
  • L1 -L2: 2 x 7628 prepreg
  • L2-L3: 0.8mm Core material 35/35
  • L3 – L4:  2 x 7628 prepreg
  • 0.5oz foil

2. Standard 6 layer PTH, 1.6mm thick, 1 oz copper all layers

  • 0.5oz foil
  • L1 -L2: 2 x 7628 prepreg
  • L2-L3: 0.5mm Core material 35/35
  • L3 – L4:  2 x 7628 prepreg
  • L4-L5: 0.5mm Core material 35/35
  • L5 – L6:  2 x 7628 prepreg
  • 0.5oz foil

Overall board thickness can be adjusted by using thicker or thinner cores as well as 1080 prepreg.  When calculating specific layer dielectrics, tolerances of materials must be factored in as there are small compounded variances right through the layup process.  Standard materials available in Australia are Class C which have a thickness tolerance of +/- 10%.

Even or matching dielectrics layer to layer reduces warp and provides the foundation for good reliable multi-layer pcbs.  Using two pieces of prepreg between layers reduces the risk of plane to plane short circuits that can occur if the resin flows too much, creating a connection in the open part of the weave.