PCBFast

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 2 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. We do not make blind/buried via jobs locally.

Nominal thickness of prepregs carried in stock:

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

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 epoxy 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 the 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 +/-0.15mm, remembering that board thickness is measured over the fibreglass, not the copper.

On internal layers, we require minimum 0.5mm copper clearance from drill to plane.  Anything less than 0.5mm clearance will be rejected and advised at quote to enable a prompt correction should the job be urgent.

1. Standard 4 layer PTH Construction Stackup, 1.6mm +/- 0.15mm thick, 1 oz copper all layers = approx 1.53mm over the dielectric, assuming plane internal layers

• L1 = 18um copper foil electroplated up to achieve min ave 35um copper & 20um in PTH/vias
• L1 – L2 dielectric: 2 x 7628 prepreg = 0.33mm nominal
• L2 = 35um copper
• L2 – L3 dielectric: 0.8mm thick core
• L3 = 35um copper
• L3 – L4:  2 x 7628 prepreg = 0.33mm nominal
• L4 = 18um copper foil electroplated up to achieve min ave 35um copper & 20um in PTH/vias

2. Standard 6 layer PTH, 1.6mm +/- 0.15mm thick, 1 oz copper all layers = approx 1.5mm thick over the dielectric, assuming plane internals

• L1 = 18um copper foil electroplated up to achieve min ave 35um copper & 20um in PTH/vias
• L1 – L2 dielectric: 1 x 1080 prepreg & 1 x 7628 prepreg = 0.21mm nominal
• L2 = 35um copper
• L2 – L3 dielectric: 0.36mm core nominal
• L3 = 35um copper
• L3 – L4:   1 x 1080 prepreg & 1 x 7628 prepreg = 0.21mm nominal
• L4 – L5: dielectric: 0.36mm core nominal
• L5 = 35um copper
• L5 – L6 dielectric:  1 x 1080 prepreg & 1 x 7628 prepreg = 0.21mm nominal
• L6 = 18um copper foil electroplated up to achieve min ave 35um copper & 20um in PTH/vias

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.