PCB Tech - Copper plating process for PCB circuit board design

Eletcroless Plating Coppe (Eletcroless Plating Coppe is usually also called sinking copper or PTH) is an autocatalytic redox reaction. First, it is treated with an activator to make the insulating substrate surface adsorb a layer of active particles. Usually metal is used. Palladium particles (palladium is a very expensive metal, the price is high and has been rising, in order to reduce costs, there are practical colloidal copper processes in operation abroad), copper ions are first reduced on these active metal palladium particles, and these are The reduced metal copper crystal nucleus itself becomes the catalytic layer of copper ions, so that the reduction reaction of copper continues to proceed on the surface of these new copper crystal nuclei. Electroless copper plating has been widely used in our PCB manufacturing industry, and currently the most It uses electroless copper plating for PCB hole metallization.

The PCB hole metallization process is as follows:

Drilling + grinding plate deburring + upper plate ten whole hole cleaning treatment ten double washing + micro-etching chemical roughening + double washing one pre-soaking treatment one colloidal palladium activation treatment one double washing + degumming treatment (accelerated) + double washing + sinking Copper one pair of water wash ten lower board ten upper board + pickling eleven times copper ten water wash one board + drying

1. Pre-plating treatment

1. Deburring

After drilling the copper clad board, some small burrs will inevitably be generated at the orifice. If these burrs are not removed, the quality of the metallized holes will be affected. The easiest way to deburr is to polish the surface of the copper foil after drilling with 200~400 water sandpaper. The mechanized method of deburring is to use a deburring machine. The grinding roller of the deburring machine adopts nylon brush or felt containing silicon carbide abrasive. When the general deburring machine removes burrs, some burrs fall to the inner wall of the orifice along the moving direction of the plate surface. The improved plate grinding machine has a two-way rotating nylon brush roller with a swinging nylon brush roller, which eliminates this problem.

2. Hole cleaning treatment

There is a requirement for the whole hole of the multilayer PCB, the purpose is to remove the drilling dirt and hole micro-etching treatment. In the past, concentrated sulfuric acid was used to remove drilling dirt, but now alkaline potassium permanganate treatment is used, followed by cleaning and adjustment treatment.

When the hole is metallized, the electroless copper plating reaction occurs simultaneously on the hole wall and the entire surface of the copper foil. If some parts are not clean, it will affect the bonding strength between the electroless copper plating layer and the printed conductor copper foil, so the substrate must be cleaned before the electroless copper plating.

3. Roughening treatment of copper clad foil

The copper surface is etched by chemical micro-etching method (etching depth is 2-3 microns), so that the copper surface produces uneven micro-roughness with active surface, so as to ensure that there is a firm relationship between the electroless copper layer and the copper foil substrate The bonding strength. In the past, the roughening treatment mainly used persulfate or acid copper chloride aqueous solution for micro-etching roughening treatment. Nowadays, sulfuric acid/hydrogen peroxide (HS0/H0) is mostly used, the etching speed is relatively constant, and the roughening effect is uniform. Since hydrogen peroxide is easy to decompose, a suitable stabilizer should be added to the solution, which can control the rapid decomposition of hydrogen peroxide, improve the stability of the etching solution, and further reduce the cost.

Second, activation

The purpose of activation is to adsorb a layer of catalytic metal particles on the surface of the substrate, so that the electroless copper plating reaction can proceed smoothly on the entire substrate surface. Commonly used activation treatment methods include sensitization-activation method (step activation method) and colloidal solution activation method (one-step activation method).

Three, electroless copper plating

1. Electroless copper plating solution

At present, the most widely used formulas are several kinds of electroless copper plating solutions using different complexing agents as listed in the following table. Formula 1 is potassium sodium tartrate complexing agent. Its advantage is that the electroless copper plating solution has low operating temperature and is easy to use., But the stability is poor, the copper plating layer is brittle, the copper plating time should be controlled appropriately, otherwise the brittle copper plating layer is too thick will affect the bonding strength of the plating layer and the substrate. Formulation 2 is an EDTA2Na complexing agent, which has a high use temperature, a higher deposition rate, and a better stability of the plating solution, but the cost is higher. Formulation 3 is a double complexing agent, somewhere in between.

2. The stability of electroless copper plating solution

(1) Reasons for the instability of the electroless copper plating solution

In the presence of a catalyst, the main reactions of electroless copper plating are as follows:

In addition to the main reaction of the above formula in the electroless copper plating solution, there are also the following side reactions.

a. Disproportionation reaction of formaldehyde-under the condition of concentrated alkali, part of formaldehyde is oxidized to formic acid, and the other part is reduced to methanol. The discriminatory reaction of formaldehyde will cause excessive consumption of formaldehyde and also make the plating solution premature. The "aging" makes the plating solution unstable.

b. In the alkaline copper plating solution, formaldehyde reduces a part of Cu2+ to Cu+, and the reaction formula is

The Cu20 produced by reaction formula (5-3) is slightly soluble in alkaline solution:

Cu20+H20=2Cu++20H-(5-4)

The copper Cu+ appearing in reaction (5-4) is very prone to disproportionation reaction

2Cu+=Cu0↓+ Cu2+ 5-6)

The copper produced by reaction formula (5-5) is extremely fine particles, which are randomly dispersed in the electroless copper plating solution. These copper particles are catalytic. If these copper particles are not controlled, it will quickly lead to the entire The decomposition of the plating solution is the main reason for the instability of the electroless copper plating solution.

(2) Measures to improve the stability of electroless copper plating solution

a. Add stabilizer The added stabilizer has a very strong complexing ability to Cu+, but poor complexing ability to Cu2+ ions in the solution. The Cu+ ions in this solution cannot produce a disproportionation reaction, so it can stabilize the chemistry The role of copper plating solution. The stabilizer added is generally a compound containing sulfur or N. For example: a, a'bipyridine, potassium ferrocyanide, 2,9 dimethyl phenanthroline, thiourea, 2-mercaptobenzothiazole, etc.

b. In the process of electroless copper plating with air stirring, the solution is stirred with air, which can inhibit the production of Cu20 to a certain extent, thereby stabilizing the solution. c. Continuous filtration The chemical copper plating solution is continuously filtered with a filter element with a particle size of 5 pm, which can filter out the active particulate matter in the plating solution at any time.

d. Adding polymer compounds to mask copper particles. Many polymer compounds containing hydroxyl and ether groups can be adsorbed on the surface of copper. In this way, the copper particles produced due to the disproportionation reaction of Cu20 will lose their catalytic performance after adsorbing these polymer compounds on their surface and no longer play the role of decomposing the solution. The most commonly used polymer compounds are polyethylene glycol, polyethylene glycol sulfide and so on.

e. Work load control. Different electroless copper plating baths have different workloads. If "overload" occurs, the decomposition of the electroless copper plating bath will be accelerated. The working load of the electroless copper plating solution listed in Table 4 is generally not greater than 1dm2/L during continuous work.

3. Toughness of the electroless copper layer

In order to ensure the reliability of the connection of the PCB metallization holes, the electroless copper layer must have sufficient toughness. The main reason for the poor toughness of the electroless copper layer is due to the release of hydrogen when formaldehyde reduces Cu2. Although hydrogen cannot be co-deposited with copper, in the copper plating reaction, the hydrogen will be adsorbed on the surface of the copper and accumulate bubbles into the copper plating layer, causing a large number of bubble cavities in the copper plating layer. These cavities will cause chemical The resistance of the copper plating layer becomes higher and the toughness becomes worse.

The main measure to improve the toughness of the electroless copper layer is to add a hydrogen barrier agent in the plating solution to prevent hydrogen from accumulating on the surface of the copper layer.