Products with a three-layer PCB or more are referred to as multilayer PCBs. Traditional double-sided boards are densely packed with components. Due to the limitation of board surface area, it is not feasible to accommodate a large number of components and the extensive routing on a single layer. Thus, multiple layers are used in the development of PCB boards.
Originally, four-layer PCBs were commonly upgraded to six-layer PCBs. As high-density assembly continues to advance, even more layers are becoming standard in high-level multilayer PCBs. This chapter will focus on the production process and considerations for the inner layers of multilayer PCBs.
**Production Process**
Depending on the specific product, there are three main processes:
A. **Print and Etch**
Sending – Alignment hole – Copper surface treatment – Image transfer – Etching – Stripping
B. **Post-etch Punch**
Sending – Copper surface treatment – Image transfer – Etching – Peeling film – Tool hole
C. **Drill and Panel-plate**
Sending – Drilling – Through Hole – Plating – Image Transfer – Etching – Stripping
**Issue**
Material preparation involves cutting the substrate according to the BOM and the working size specified in the pre-production design. Although this step is straightforward, attention must be given to the following points:
A. Cutting method – This affects the cutting dimensions.
B. Considerations of edging and rounding – These impact the yield of the image transfer process.
C. The direction should be consistent—specifically, the warp direction should be aligned with the warp direction, and the latitude direction should be aligned with the weft direction.
D. Baking before proceeding to the next process is essential for ensuring dimensional stability.
Copper Surface Treatment
In the printed circuit board manufacturing process, every step involving the cleaning and roughening of the copper surface significantly impacts the success of subsequent processes. Although this may seem straightforward, it involves considerable expertise.
A. Processes that require copper surface treatment include:
a. Dry film pressing
b. Inner layer oxidation treatment
c. Post-drilling
d. Pre-chemical copper application
e. Pre-copper plating
f. Pre-green paint application
g. Pre-solder pad processing (e.g., tin spraying)
h. Gold finger treatment before nickel plating
This section covers the optimal methods for processes a, c, f, and g (the remaining processes are part of automation and do not require independent discussion).
B. Treatment Methods
Current copper surface treatment methods can be categorized into three types:
a. Brushing
b. Sandblasting (Pumice)
c. Chemical (Microetch)
The following provides an overview of these three methods:
**Brushing**
a. Ensure the brush wheel’s effective length is used uniformly to prevent uneven surface height.
b. Conduct brush mark experiments to assess the depth and uniformity of the brush marks.
c. Advantages:
– Low cost
– Simple manufacturing process
d. Disadvantages:
– Difficulties with thin circuit boards
– Elongation of the base material, unsuitable for inner layers
– Deep brush marks can lead to D/F (dielectric-to-foil) adhesion issues and infiltration
– Potential for residual glue
**Sandblasting**
Using fine stones of various materials (commonly known as pumice) for abrasion has the following benefits:
a. Superior surface roughness and uniformity compared to brushing
b. Better size stability
c. Suitable for thin plates and wires
Disadvantages:
a. Pumice can adhere to surfaces
b. Maintenance of the machine can be challenging
**Chemical Method (Microetching)**
a. Provides detailed information on image transfer
b. Outlines the printing methods used in conjunction with microetching
Originally, four-layer PCBs were commonly upgraded to six-layer PCBs. As high-density assembly continues to advance, even more layers are becoming standard in high-level multilayer PCBs. This chapter will focus on the production process and considerations for the inner layers of multilayer PCBs.
**Production Process**
Depending on the specific product, there are three main processes:
A. **Print and Etch**
Sending – Alignment hole – Copper surface treatment – Image transfer – Etching – Stripping
B. **Post-etch Punch**
Sending – Copper surface treatment – Image transfer – Etching – Peeling film – Tool hole
C. **Drill and Panel-plate**
Sending – Drilling – Through Hole – Plating – Image Transfer – Etching – Stripping
**Issue**
Material preparation involves cutting the substrate according to the BOM and the working size specified in the pre-production design. Although this step is straightforward, attention must be given to the following points:
A. Cutting method – This affects the cutting dimensions.
B. Considerations of edging and rounding – These impact the yield of the image transfer process.
C. The direction should be consistent—specifically, the warp direction should be aligned with the warp direction, and the latitude direction should be aligned with the weft direction.
D. Baking before proceeding to the next process is essential for ensuring dimensional stability.
Copper Surface Treatment
In the printed circuit board manufacturing process, every step involving the cleaning and roughening of the copper surface significantly impacts the success of subsequent processes. Although this may seem straightforward, it involves considerable expertise.
A. Processes that require copper surface treatment include:
a. Dry film pressing
b. Inner layer oxidation treatment
c. Post-drilling
d. Pre-chemical copper application
e. Pre-copper plating
f. Pre-green paint application
g. Pre-solder pad processing (e.g., tin spraying)
h. Gold finger treatment before nickel plating
This section covers the optimal methods for processes a, c, f, and g (the remaining processes are part of automation and do not require independent discussion).
B. Treatment Methods
Current copper surface treatment methods can be categorized into three types:
a. Brushing
b. Sandblasting (Pumice)
c. Chemical (Microetch)
The following provides an overview of these three methods:
**Brushing**
a. Ensure the brush wheel’s effective length is used uniformly to prevent uneven surface height.
b. Conduct brush mark experiments to assess the depth and uniformity of the brush marks.
c. Advantages:
– Low cost
– Simple manufacturing process
d. Disadvantages:
– Difficulties with thin circuit boards
– Elongation of the base material, unsuitable for inner layers
– Deep brush marks can lead to D/F (dielectric-to-foil) adhesion issues and infiltration
– Potential for residual glue
**Sandblasting**
Using fine stones of various materials (commonly known as pumice) for abrasion has the following benefits:
a. Superior surface roughness and uniformity compared to brushing
b. Better size stability
c. Suitable for thin plates and wires
Disadvantages:
a. Pumice can adhere to surfaces
b. Maintenance of the machine can be challenging
**Chemical Method (Microetching)**
a. Provides detailed information on image transfer
b. Outlines the printing methods used in conjunction with microetching
