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Understanding PCB Manufacturing: Multilayer Assembly

As an aid to multilayer board design, we are able to provide thickness specifications for some standard materials.

Step 1: Understanding PCB Manufacturing: Multilayer Assembly

Manufacturing process for multilayer_ PCBs

Manufacturing multilayer_ circuit boards is a straightforward process but does require a high level of attention to detail. Care must be taken to ensure all layers are correctly registered to the required drilled holes despite the deformation stresses produced by the heat and pressure. The buildup requires the operator ensures correct materials are chosen, the build sequence is followed and the orientation of each sheet is correct. Each unpressed PCB panel is loaded as a “chapter” with up to 10 chapters pressed together to form a “book” separated by heavy steel plates. These books are then loaded into each chamber of a hydraulic press. At pcbway, we have the capacity to press up to 30 PCB panels in each load. The process for pressing polyimide material is modified slightly from the process required for FR4.

Specialized equipment for pressing multilayer_

PCBs Manufacturing multilayer_ PCBs requires a specialized hydraulic press with heated platens. Initially the books are squeezed with a “kiss” pressure of 50 psi prior to being heated to 350F at 350 psi for a minimum of one hour. The assembly is then allowed to cool slowly before removal for further processing.

Design for Manufacturability (DFM) for multilayer_ circuit boards

A few considerations will help ensure manufacturable PCB designs.
Copper areas - on inner layers, all copper should be kept at least 10 mil from the outer edge of the board, 20 mil is preferred.
Via clearance through inner layers (antipads)
Allow for clearance around any holes or via barrels not connected to an inner layer. This clearance should be at least 15 mil although 20 mil clearance is preferred.
Thermal Relief Pads - the tie should be a minimum of 8 mil while more is preferable.
Larger geometries - will result in higher yields which will be reflected in your board price.

As an aid to multilayer_ board design, we are able to provide thickness specifications for some standard materials.

Multilayer_ Thickness Parameters

Copper Foil (Outer Layers)	Approximate Thickness (mils)
0.5 oz Cu foil	0.67
1.0 oz Cu foil	1.38
2.0 oz Cu foil	2.76

Electroplate (Outer Layers)
0.5 oz Cu	0.67
0.7 oz Cu	1.00
1.0 oz Cu foil	1.38
1.5 oz Cu	2.07
2.0 oz Cu foil	2.76

Prepreg Materials
NP175FB 2116 (cured*)	4.1
NP175FB 7628MR (cured*)	7.5

Core Materials
NP175FTL 0.005 H/H Cu	6.2
NP175FTL 0.010 H/H Cu	10.8
NP175FTL 0.010 1/1 Cu	12.2
NP175FTL 0.014 H/H Cu	15.0
NP175FTL 0.014 1/1 Cu	16.4
NP175FTL 0.021 1/1 Cu	23.5
NP175FTL 0.028 1/1 Cu	30.5

*between solid foils (no void filling)

Bow and Twist in multilayer_ PCBs is typically the result of unconventional designs.

Bow and twist is more likely to occur in asymmetric designs which can result in unbalanced stress conditions. For example, odd layer counts (3, 5 layer) are known to cause issues. Another source of multilayer_ PCB bow and twist comes from designs which specify variable layer thicknesses. For example, a 4 layer build specification of 7 / 28 / 21 creates more risk of deformation than a standard build. Even different circuit configurations can be influencing factors.

Final thickness standards for multilayer_ builds.

Final thickness varies somewhat depending on circuit patterns. Solid copper planes yield thicker boards than layers with little or no copper. Thickness tolerance is typically +/- 10%.

Multilayer_ Thickness Parameters

Standard Multilayer_ Builds	Mils per Dielectric Layer	Resulting Thickness
4 Layer	14 / 28 / 14	62 mil
6 Layer	7 / 14 / 14 / 14 / 7	62 mil
8 Layer	7 / 5 / 11 / 5 / 11 / 5 / 7	62 mil
10 Layer	4 / 5 / 7 / 5 / 7 / 5 / 7 / 5 / 4	62 mil