Electroless copper plating (Eletcroless Plating Coppe, also commonly known as copper sinking or porosity (PTH) is an autocatalytic redox reaction. First, it is treated with an activator to adsorb a layer of active particles on the surface of the insulating substrate. Metal is usually 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 nuclei themselves become the catalytic layer of copper ions, so that the reduction reaction of copper continues on the surface of these new copper nuclei. Electroless copper plating has been widely used in our PCB manufacturing industry, and currently the most It is the hole metallization of PCB with electroless copper plating.
The PCB hole metallization process flow is as follows:
Drilling + grinding plate deburring + upper plate ten whole hole cleaning treatment ten double water washing + micro-etching chemical roughening + double water washing - pre-dipping treatment - colloidal palladium activation treatment - double water washing + degumming treatment (acceleration) + double water washing + sinking A pair of copper washes ten lower boards and ten upper boards + pickling eleven times copper ten water wash the boards + drying.
1. Pre-plating treatment
1.1. Deburring: After drilling the CCL, some small burrs are inevitably 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 drilled copper foil with 200~400 grit water sandpaper. The mechanized deburring method is to use a deburring machine. The grinding roller of the deburring machine is a nylon brush or felt containing silicon carbide abrasive. When the general deburring machine removes the burr, 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 bidirectional rotating belt with a swinging nylon brush roller, which eliminates this drawback.
1.2. Hole cleaning treatment: There are whole hole requirements for multi-layer PCB, the purpose is to remove drilling dirt and hole micro-etching treatment. In the past, concentrated sulfuric acid was used to remove drill pollution, but now, alkaline potassium permanganate treatment is often used, followed by cleaning and adjustment. During hole metallization, the electroless copper plating reaction occurs simultaneously on the hole wall and the entire copper foil surface. 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.
1.3. 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 an uneven micro-rough and active surface, so as to ensure chemical copper plating There is a strong bond strength between the layer and the copper foil substrate. In the past roughening treatment, persulfate or acidic copper chloride aqueous solution was mainly used for micro-etching roughening treatment. At present, sulfuric acid/hydrogen peroxide (HS0/H0) is mostly used, and its etching speed is relatively constant, and the roughening effect is uniform. Since hydrogen peroxide is easily decomposed, 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.
2. Activation
The purpose of activation is to adsorb a layer of catalytic metal particles on the surface of the substrate, so that the entire surface of the substrate can smoothly carry out the electroless copper plating reaction. Commonly used activation treatment methods include sensitization-activation method (step-by-step activation method) and colloidal solution activation method (one-step activation method).
3. Electroless copper plating
3.1. Electroless copper plating solution
At present, the most widely used formulas are the chemical copper plating solutions listed in the following table using different complexing agents. Formula 1 is a complexing agent of potassium and sodium tartrate, which has the advantages of low operating temperature and convenient use. , but the stability is poor, the copper plating layer is brittle, and the copper plating time should be properly controlled, otherwise the brittle copper plating layer will be too thick, which will affect the bonding strength between the plating layer and the substrate. Formula 2 is an EDTA2Na complexing agent, which has a high use temperature, a high deposition rate, and good stability of the plating solution, but the cost is high. Formulation 3 is a double complexing agent, in between.
3.2. Stability of electroless copper plating solution
(1) The reason for the instability of the electroless copper plating solution
In the presence of a catalyst, the main reaction of electroless copper plating is as follows. In addition to the main reaction of the above formula, the following side reactions also exist in the electroless copper plating solution.
a. Disproportionation reaction of formaldehyde - under the condition of concentrated alkali, a part of formaldehyde is oxidized to formic acid, and the other part is reduced to methanol. The discriminative reaction of formaldehyde not only causes excessive consumption of formaldehyde, but also makes 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 reaction formula (5-3). The Cu20 generated is slightly soluble in the alkaline solution: Cu20+H20=2Cu++20H -(5-4) The copper Cu+ that appears in the reaction (5-4) is very prone to disproportionation reaction 2Cu+=Cu0↓+ Cu2+ 5-6) The copper produced by the reaction formula (5-5) is extremely fine particles, they Dispersed randomly in the electroless copper plating solution, these copper particles have catalytic properties. If these copper particles are not controlled, the entire plating solution will be rapidly decomposed, which 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. The stabilizer added by the stabilizer has a strong complexing ability to Cu+, but poor complexing ability to Cu2+ ions in the solution. The role of copper plating solution. The added stabilizer is generally a sulfur or N-containing compound. For example: a, a' bipyridine, potassium ferrocyanide, 2,9 dimethyl phenanthroline, thiourea, 2-mercaptobenzothiazole, etc.
b. In the process of electroless copper plating with gas stirring, stirring the solution with air can inhibit the generation of Cu20 to a certain extent, thus playing the role of stabilizing the solution.
c. Continuous Filtration Use a filter element with a particle size of 5pm to continuously filter the electroless copper plating solution, 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 generated due to the disproportionation reaction of Cu20 will lose their catalytic performance after adsorbing these polymer compounds on their surfaces, and will no longer play the role of decomposing the solution. The most commonly used polymer compounds are polyethylene glycol, polyethylene glycol sulfide, etc.
e. Controlling the workload has different workloads for different electroless copper plating solutions, if "overload" will accelerate the decomposition of the electroless copper plating solution. The working load of the electroless copper plating solution listed in Table 4 should generally not be greater than 1dm2/L during continuous operation.
3.3. Toughness of electroless copper plating layer
In order to ensure the reliability of the PCB metallized hole connection, the electroless copper layer must have sufficient toughness. The main reason for the poor toughness of the electroless copper plating layer is due to the release of hydrogen gas when formaldehyde reduces Cu2. Although hydrogen gas cannot be co-deposited with copper, in the copper plating reaction, these hydrogen gas will be adsorbed on the surface of copper, aggregated into bubbles and mixed in the copper plating layer, resulting in a large number of bubble voids in the copper plating layer, which will cause chemical The electrical resistance of the copper plating layer becomes high, and the toughness becomes poor.
The main measure to improve the toughness of the electroless copper plating layer is to add a hydrogen blocking agent to the plating solution to prevent hydrogen from accumulating on the surface of the copper layer. The following table lists the effect of a, a'' bipyridine and other additives on the toughness of the copper plating layer of the electroless copper plating solution with EDTA as a complexing agent. The copper plating temperature is 70C, and the a, a' bipyridine The added amount is 100 mg/L.
3.4. Deposition rate of electroless plating solution
3.5. Automatic analysis and automatic replenishment of electroless copper plating solution
3.6. Common defects and solutions
(1) The bonding force between the electroless copper plating layer and the copper foil is poor
a. The surface roughening treatment of copper foil is not enough;
b. Poor cleaning of the surface of the substrate after roughening;
c. The electroless copper plating layer is too brittle.
(2) The copper-plated layer of the metallized hole wall has pinholes
a. The drilling quality is too poor, because the drill bit is not sharp, a large amount of foil clad sheet chips remain on the hole wall or in the residual treatment liquid during the drilling process, so that no copper is deposited in these parts;
b. The activation treatment is poor, the activation solution is not active enough, the temperature is too low, the hole is not clean, etc.;
c. The pH of electroless copper plating is too low;
d. Insufficient air agitation does not drive off the hydrogen adsorbed during copper reduction.
(3), the appearance of electroless plating is black
a. The composition ratio of chemical copper plating solution is unreasonable;
b. The process operating conditions are not strictly controlled;
c. The plating solution load is too large
Shenzhen pinban Technology Co., Ltd. is a manufacturer specializing in PCB boards, FPC boards, and membrane switch panels. The main products are: PCB circuit board, FPC flexible circuit board, membrane switch, membrane panel, acrylic panel and other products, the company has a vigorous, professional and experienced technical, production and management team, with advanced printing The special production and testing equipment for boards has a standardized operation system, which makes the internal operation more accurate and efficient, and the external service is more systematic and timely, creating greater value for customers.
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