Multi-Layer PCBs

Standard manufacturing of PTH via 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 2 layer process with the addition of a special cleaning treatment after drill.

Please note the following:

  • We DO NOT manufacture blind &/or buried via jobs in our local Australian factory.
  • We ONLY use standard 140°C Tg FR4 laminate and prepregs.

Nominal thickness of the prepreg styles we carry in stock:

  • 7628 – 6.5thou or 0.165mm, +/-10%
  • 1080 – 2.5thou or 0.0635mm, +/-10%

Tolerances on multi-layer finished thickness is typically +/-10%.

On internal layers, we require minimum 0.45mm copper clearance from drill to plane.  Anything less than 0.45mm clearance will be rejected and advised at quote to enable a prompt correction should the job be urgent.

Our typical layer stack ups for 1.6mm thick 4 layer PCBs, unless there is a specific requirement in the design, are below.

A. 4 layer PTH Construction Stackup, 1.6mm +/- 0.15mm thick, 1 oz copper all layers

  • L1 = 18um copper foil electroplated up to achieve min  20-25µm in PTH/vias = ave 40 – 50µm average surface copper depending on the track layout
  • L1 – L2 dielectric: 2 x 7628 prepreg = 0.33mm nominal
  • L2 = 35µm copper
  • L2 – L3 dielectric: 0.8mm thick core
  • L3 = 35µm copper
  • L3 – L4:  2 x 7628 prepreg = 0.33mm nominal
  • L4 = 18um copper foil electroplated up to achieve min  20-25µm in PTH/vias = ave 40 – 50µm average surface copper depending on the track layout

B. 4 layer PTH Construction Stackup, 1.6mm +/- 0.15mm thick, 1 oz copper all layers

  • L1 = 18um copper foil electroplated up to achieve min  20-25µm in PTH/vias = ave 40 – 50µm average surface copper depending on the track layout
  • L1 – L2 dielectric: 1 x 7628 prepreg + 1 x 1080 = 0.23mm nominal
  • L2 = 35µm copper
  • L2 – L3 dielectric: 1.0mm thick core
  • L3 = 35µm copper
  • L3 – L4:  1 x 7628 prepreg + 1 x 1080 = 0.23mm nominal
  • L4 = 18um copper foil electroplated up to achieve min  20-25µm in PTH/vias = ave 40 – 50µm average surface copper depending on the track layout

C. 4 layer PTH Construction Stackup, 1.6mm +/- 0.15mm thick, 1 oz copper all layers

  • L1 = 18um copper foil electroplated up to achieve min  20-25µm in PTH/vias = ave 40 – 50µm average surface copper depending on the track layout
  • L1 – L2 dielectric: 2 x 1080 prepreg = 0.127mm nominal
  • L2 = 35µm copper
  • L2 – L3 dielectric: 1.2mm thick core
  • L3 = 35µm copper
  • L3 – L4 dielectric: 2 x 1080 prepreg = 0.127mm nominal
  • L4 = 18um copper foil electroplated up to achieve min  20-25µm in PTH/vias = ave 40 – 50µm average surface copper depending on the track layout

Standard core thicknesses in stock include: 0.25mm, 0.3mm, 0.5mm, 0.8mm, 1.0mm & 1.2mm.  All cores are 1oz copper but can be plated up for 2oz.

Overall board thickness and dielectric thickness can be adjusted by using thicker or thinner cores as well as different prepreg selections.  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 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.  The final thickness of any dielectric using prepreg will depend on the style used, the number of pieces used and the location of the pieces in the stack.