Multilayer
Foil Lamination is the method of choice assigned by GPV Printca. It is the most cost effective manufacturing process and minimises potential for bow and twist. For SPACE applications laminate constructions are used due to surface quality.
If specifying dielectric thickness, as may be required for impedance reasons for example, the dimensions should be selected from core or prepreg thicknesses that are available from GPV.
Dielectric thicknesses made up of prepreg depend on the type or the combination of different types of these materials. GPV will advise you of what combination of prepreg is suitable and of achievable dimensions and tolerances.
It is beneficial to discuss special dielectric requirements with GPV Printca during the design stage if possible. This will allow time for material procurement if necessary. Also, manufacturing concerns can be addressed while an opportunity still exists to make changes.
Thickness is not the only indicator of material cost. Other factors, such as type, thickness tolerance, or the demand for this material may influence cost. If no specific dielectric thicknesses are required, it is best to allow GPV to make the material selection. Materials that will be utilised will meet industry standards, be of lowest cost, and allow the most effective manufacturing methods.
Maintaining a balanced lay-up in relation to the Z-axis median of the board will assure minimum bow and twist. This balance includes the following:
• Dielectric thickness of layer
• Copper thickness of layers and its distribution
• Location of circuit and plane layers
• Glass-styles used in laminates and prepregs
A higher number of layers normally means an increase number of plane layers. Planes need to be balanced around the Z-axis median line of the lay-up, and ideally located internal to the board.
If accepted multilayer design rules are adhered to, boards will meet a maximum allowable bow and twist specification of 1% or better. GPV would use thick laminate cores, min. 2 sheet prepregs between layers, only dimensional stable laminates. When using laminate constructions, the outer laminate is used for layer 1-0, to avoid shifted layers.
Recommended materials
GPV Printca recommend to use 5, 8, 14, 22, 28 and 36 mils laminates for MLBs.
| Prepreg thickness mils: | Resin type | |||||||
| Glass-style 1080 2113 2125 2116 7628 |
GF 0.0026 0.004 0.007 |
BT 0.003 0.0046 |
GI 0.0031 0.0043 |
GIJ 0.0033 0.0046 | ||||
Outer Layer Circuitry
Circuit area and distribution between the front and back of the board should be balanced as closely as possible.
Plating thieving of low pattern density and cross hatching of external plane area should be considered.
Thickness Calculation and Tolerance
As the overall thickness of a multilayer board increases, the thickness tolerance should also increase. A good rule is to specify a tolerance of +/- 10% of the overall thickness. Always indicate where the thickness measurement is to be taken. Examples might be: glass to glass at rail guides, over gold contacts, over solder mask, etc.
When calculating the potential board thickness, consideration needs to be given to certain design characteristics. An isolated .006 inch line may totally embed itself into the prepreg and make no contribution to the thickness of the board.
Pressed thickness is calculated as:
laminates (ML laminates is measured without copper foil)
+ prepreg pressed thickness
+ Copper thickness on planes
+ Outer layer copper foil
Filling of planes with resin
When having 70µ copper planes or thicker, it should be taken into account that there is resin to fill the openings in the plane, and when combined with polyimide resin system, GIL should be changed to GIJ when filling properties is needed.
Manufacturing Drawing
The designer needs to specify the critical features of the design, i.e., finished board thickness, minimum dielectric spacing, number of layers and any electrical performance characteristic critical to the manufacture of the board, i.e., impedance requirements on the manufacturing drawing. The manufacturer should be left with the maximum amount of latitude the design will allow.
STANDARD MULTILAYER BUILD-UP´S:
| Cu. Base mat. | Layer | Plane | |
| 4-layers | |||
| 17 µ | Foil | 1 | |
| 2 x 7628 | Prepreg | ||
| 35 µ | 2 | ||
| 0.036" | Laminate | ||
| 35 µ | 3 | ||
| 2 x 7628 | Prepreg | ||
| 17 µ | Foil | 4 | |
| Pressed thickness 1.64 mm | |||
| Cu. Base mat. | Layer | Plane | |
| 4-layers with 2 plane layers | |||
| 17 µ | Foil | 1 | |
| 1 x 7628 1 x 2125 |
Prepreg Prepreg |
||
| 35 µ | |||