Initial Processes before manufacturing of PCB
- Obtaining the design of the PCB from the customer (It is called Gerber) The data consists of-
- The position of the various tracks, holes.
- The connections required between various points in different layers.
- The outline, various markings and identifications to be made on the PCB.
- The pallet requirements, their orientation, dimensions.
- The number of layers and the build up.
- Study of the customer data & its manufacturability.
- If manufacturable, then penalization, location of mounting & other tooling holes.
- Generation of drill program, test data, films.
Multi-Layer Production Flow-chart
Materials for Multilayer PCB Fabrication
Major raw materials which goes into a fabrication of a multilayer board are:
- Copper foil
- Inner cores
Major consumables in the process are:
- Kraft papers
- SS plates
- Caul plates
- Aluminium foils
Together & to provide insulation between 2 layers. I- is also necessary to provide the required thickness.–
- Prepregs are semi-cured materials. They are also known as ‘B-Stage’ materials. This is different from the ‘ A-Stage’ materials in the sense that they are not sticky. In the press, they are fully cured to mak ‘ C-Stage’ materials.
- Prepregs are glass-cloth impregnated with resin.
Prepreg – Glass Cloth
Glass cloth performs 3 major functions:
- Gives mechanical strength to the board.
- Acts as a vehicle to carry or contain resin.
- Allows the manufacturer to build the boards to a total thickness with some reasonable tolerance.
Prepreg – Resin
Epoxy is the most common resin used to manufacture multilayer boards.
Advantages of epoxy resins:
Easy to machine, laminate and process chemically.
Exhibits sufficient dimensional stability for most of the applications.
Cost effective for the most board designs.
Prepregs are classified based on resin content, resin flow and thickness.
Resin content is a measure of the weight percent of resin coated onto the reinforcement fabric. For example Resin contents of the prepregs.
A minimum amount of flow is required during multilayer lamination to encapsulate & bond the etched circuitry & ensure void free laminations. For example Resin flow of prepregs.
Cured thickness of prepregs is the major factor for determining the final thickness of board. For example Thickness of prepregs:
7628 7.2 mil
2116 4 mil
1080 2.8 mil
Prepreg Volatile Content
The residual solvent or moisture can increase resin flow, affect the rate of cure, and at higher concentrations cause voiding, blistering, and even interlaminar delamination.
The rate of moisture absorption is a function of time, relative humidity, and prepreg resin content.
Recommended volatile contents are 0.5wt% or less for epoxy resins.
Prepreg Gel Time
It is the time taken by the prepreg to change from liquid state to gel state when heated.
The gel time of the prepreg must be sufficiently long to ensure good encapsulation of the circuitry and allow volatiles and air to escape, preventing internal voids.
An excessively long gel time can cause heavy flow and lead to resin starvation and a weakened interlaminar bond.
Storage of Prepregs
The advancement or curing of prepreg continues from the time of manufacture until it is used for lamination.
The rate of advancement is a function of storage temperature and relative humidity.
The shelf life of prepregs is 6 months when stored at 4.5 deg C & it reduces to 3 months when stored at 21 deg C and at relative humidity between 30 and 50%.
Copper foils are made by an electroplating process in which the copper is plated on to a polished drum- shaped cathodes which are partially immersed in plating solution and then peeled off as a foil. The side against the drum is smooth; the other side is relatively rough and makes a better bond for prepreg resin systems.
This bond can be further enhanced by plating the copper foil with proprietary metal compositions on one or both sides. The different foil thicknesses are obtained by varying current density & drum speed.
Copper foil Manufacturing Process
Cerra Systems Printed Circuit Board Fabrication capabilities support high-frequency PCB, High-temperature Boards, Thick PCB, ultra-thin PCB, heavy copper PCB, Metal Core PCB, HDI boards with Blind Vias, Buried Vias, Micro Vias, Embedded passives, bonded heat sink, Impedance Control, Depth control drilling, Back drilling, Edge Plated PCB, Bump Pads, cavity with ledge, Via on pad and stacked micro-Via technology.
CerraSystems – http://cerrasystems.com
HDI PCB , Rigid Flex PCB, RF PCB
Electronics Manufacturing Partners:
Argus Systems (AESPL) – www.sysargus.com