Detailed Explanation of The Copper Sinking Process of PCB Processing

Process introduction and technical point analysis of circuit board copper sinking process:

Electroless copper is widely used in the production and processing of printed circuit boards with through holes. Its main purpose is to deposit a layer of copper on a non-conductive substrate through a series of chemical treatment methods, and then thicken it through subsequent electroplating methods. To achieve the specific thickness of the design, it is generally 1mil (25.4um) or thicker, and sometimes it is even directly deposited to the entire copper thickness of the circuit by chemical methods. The electroless copper process is to finally complete the deposition of electroless copper through a series of necessary steps, each of which is important to the entire process flow.

The purpose of this chapter is not to describe the production process of circuit boards in detail, but to emphasize some points about chemical copper deposition in the production of circuit boards.

The concept of plated through holes (metallized holes) includes at least one or both of the following two meanings:

1. Form part of the conductor circuit of the element

2. The formation of interlayer interconnection lines or printed lines

A general circuit board is etched (on a copper-clad substrate) or on a non-conductive composite substrate (epoxy-glass fiber cloth substrate, phenolic paper substrate, polyester glass fiber board, etc.) It is produced and processed by the method of electroless plating (on the copper clad substrate or the copper foil substrate).

PI polyimide resin substrate: used for flexible board (FPC) production, suitable for high temperature requirements.

Phenolic paper substrate: can be stamped, NEMA grade, such as: FR-2, XXX-PC.

Epoxy paper substrate: better mechanical properties than phenolic cardboard, NEMA grade, such as: CEM-1, FR-3.

Epoxy glass fiber board: glass fiber cloth is used as reinforcement material, with excellent mechanical properties, NEMA grade, such as: FR-4, FR-5, G-10, G-11.

Non-woven glass fiber polyester substrate: suitable for some special purposes, NEMA grade, common such as: FR-6.

Chemical Copper/Immersion Copper:

The holes in the non-conductive substrate can be metallized for better solderability in interlayer interconnects or assembly, or both. There may be inner layers inside the non-conductive substrate – the circuit has been etched before the non-conductive substrate is laminated, and the board processed by this process is also called a multi-layer board (MLB). In multilayer boards, metallized holes not only serve to connect the two outer layers, but also to interconnect the inner layers, adding holes designed to pass through the non-conductive substrate.

Now raw rubbing and many circuit boards use laminate substrate blanking in terms of process characteristics, that is to say, the outside of the non-conductor substrate is a copper foil made by lamination to a certain thickness by electrolysis. The thickness of copper foil is expressed by the weight of copper foil per square foot (ounces), which is converted into thickness as shown in Table 13.1: these methods generally use fine abrasives such as glass beads or alumina to grind Materials. In the wet slurry process, the holes are treated with spray nozzles. Some chemical materials are used to dissolve the polymer resin either in the etch back and/or desmear process. Usually (eg epoxy resin systems), concentrated sulfuric acid is used, chromic acid aqueous solution, etc. have been used. No matter which method, good post-treatment is required, otherwise it may cause many problems such as poor deposition of chemical copper in subsequent wet process perforation.

Chromic Acid Method:
The presence of hexavalent chromium in the hole will cause many problems in the coverage of chemical copper in the hole. It will destroy the tin-palladium colloid through the oxidation mechanism and hinder the reduction reaction of chemical copper. This situation can be solved by secondary activation, but the cost of rework or secondary activation is too high, especially in automatic lines, and the secondary activation process is not very mature.
After the chromic acid tank treatment, there is often a neutralization step. Generally, sodium bisulfite is used to reduce hexavalent chromium to trivalent chromium. Generally at 120-150F, sulfite can be cleaned to avoid other bath liquids brought into the process and interfere with activation.

Concentrated Sulfuric Acid Method:
After the bath is treated, there should be a very good washing, preferably hot water, and try to avoid strong alkaline solution during washing. It may form some sodium salt residues of epoxy resin sulfonate, which is difficult to It is removed from the hole by cleaning. Its presence will form contamination in the hole, which may cause many plating difficulties.

Other Systems:
There are also other chemical methods used in desmear/drill and etch back processes. In these systems, the application of a mixture of organic solvents (bulking/swelling resins) and potassium permanganate treatment was also used in the post-treatment of concentrated sulfuric acid treatment, and now even directly replaces the concentrated sulfuric acid method/chromic acid method.
In addition, there is the plasma method, which is still in the experimental application stage, which is difficult to be used for large-scale production, and the equipment investment is relatively large.