PCB Technical - PCB inner layer production and inspection skills

Products with a three-layer PCB or more are called multilayer PCBs. The traditional double-sided board is a dense assembly of matching parts. It is impossible to place so many components and a large number of lines derived from it on a limited board surface, so there are multiple layers. The development of PCB boards.

In addition, the US Federal Communications Commission (FCC) announced that since October 1984, all electrical products on the market must be "grounded" to eliminate the effects of interference if they involve telex communicators or those who participate in network connections. However, due to insufficient board area, PCBlay-out moved the large copper surface with the two functions of "grounding" and "voltage" into the inner layer, resulting in the rapid rise of four-layer PCB boards, which also extended the impedance control requirements.

The original four-layer PCB boards are mostly upgraded to six-layer PCB boards. Of course, high-level multilayer PCB boards are also increasing due to high-density assembly. This chapter will discuss the inner layer production and precautions of multilayer PCB boards.

Production process

According to different products, there are three kinds of 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.Drilland Panel-plate

Sending - Drilling - Through Hole - Plating - Image Transfer - Etching - Stripping

Issue

Sending the material is to cut the substrate according to the BOM according to the working size planned by the pre-production design. It is a very simple step, but the following points must be paid attention to:

A. Cutting method-will affect the cutting size

B. Considerations of edging and rounding-affecting the image transfer yield process

C. The direction should be the same-that is, the warp direction is opposite to the warp direction, and the latitude direction is to the weft direction

D. Baking before the next process-consideration of dimensional stability

Copper surface treatment

In the printed circuit board manufacturing process, no matter which step, the effect of cleaning and roughening the copper surface is related to the success or failure of the next process, so it seems simple, but in fact there is quite a lot of knowledge.

A. The processes that require copper surface treatment are as follows

a. Dry film pressing

b. Before the inner layer oxidation treatment

c. After drilling

d. Before chemical copper

e. Before copper plating

f. Before the green paint

g. Before spraying tin (or other solder pad processing procedures)

h. Gold finger before nickel plating

This section discusses the best processing methods for processes such as a.c.f.g. (the rest are part of the process automation and do not need to be independent)

B. Treatment method

The current copper surface treatment methods can be divided into three types:

a. Brushing method

b. Sandblasting

c. Chemical method

The following is an introduction to these three methods

Brushing

a. The effective length of the brush wheel must be used evenly, otherwise it is easy to cause uneven surface height of the brush wheel

b. The brush mark experiment must be done to determine the advantages of brush depth and uniformity

a. Low cost

b. The circuit boards manufacturing process is simple, and the shortcomings of flexibility

a. Thin circuit boards are not easy to carry out

b. The base material is elongated, which is not suitable for the inner sheet

c. When the brush marks are deep, it is easy to cause D/F adhesion and infiltration

d. There is potential for residual glue

Sandblasting

The advantages of using fine stones of different materials (commonly known as pumice) as abrasive materials:

a. The surface roughness and uniformity are better than the brushing method

b. Better size stability

c. It can be used for thin plates and thin wires. Disadvantages:

a.Pumice is easy to stick to the surface

b. Machine maintenance is not easy

Chemical method (micro-etching method)

Image transfer

Printing method

Since the origin of the circuit board to the current high-density design, it has always been directly and closely related to silk screen printing-or screen printing, so it is called "printed circuit board". At present, in addition to the largest number of applications in circuit boards, other electronics industries still have thick film hybrid circuits (Hybrid CIRcuit), chip resistors (Chip Resist), and surface mounting (Surface Mounting) solder paste printing There are also excellent applications.

Due to the high-density and high-precision requirements of circuit boards in recent years, the printing method has been unable to meet the specification requirements, so its application range is gradually shrinking, and the dry film method has replaced most of the image transfer production methods. The following are still available for printing cover Process:

a. Single-sided circuit, anti-soldering (mass production mostly uses automatic printing, the same below)

b. Single-sided carbon ink or silver glue

c. Double-sided circuit, anti-soldering

d. Wet film printing

e. Large inner copper surface

f. text

g. Peelableink

In addition, the training of printing technicians is difficult, and the salary is high. The cost of the dry film method is gradually decreasing, so the growth and decline of the two are obvious.

A. Introduction to Screen Printing

Several important basic elements in screen printing: mesh, screen, emulsion, exposure machine, printing machine, squeegee, ink, oven, etc., are briefly introduced below.

a. Mesh material

(1) According to different materials, it can be divided into silk, nylon, polyester, stainless steel, etc. The most commonly used circuit boards are the latter three.

(2) Weaving method: The most commonly used and best used is Plain Weave.

(3) The relationship between mesh, thickness, diameter and opening

Opening:

Mesh number: the number of openings per inch or cm

Wire diameter: the diameter of the mesh weaving wire

Thickness: There are six thickness specifications, Slight(S), Medium(M), Thick(T), Half heavyduty(H), Heavyduty(HD), Super heavyduty(SHD)

b. The type of screen (Stencil)

(1). Direct Stencil

Coat the photosensitive latex evenly and directly on the mesh. After drying, the frame is placed on the surface of the exposure equipment and covered with the original film, and then vacuumized to make it close to the photosensitive. After development, it becomes a printable screen. . Usually the number of times the latex is applied depends on the printing thickness. This method is durable and stable, and is used for mass production. However, the production is slow, and when it is too thick, poor resolution may occur due to uneven thickness.

(2). Indirect Stencil (Indirect Stencil)

The photosensitive plate film is exposed and developed to transfer the graphics from the original film, and then the plate film with the existing graphics is pasted on the mesh surface. After the cold air is dried, the transparent carrier protective film is torn off to form an indirect network. Version. The thickness is uniform, the resolution is good, and the production is fast. It is mostly used for PCB samples and small-scale production