Analysis and treatment of common failures in plastic electroplating

Time:2021-08-20 16:15:19 / Popularity: / Source:

1. Surface of parts cannot be plated with copper layer; cause of this type of failure is generally caused by failure of sensitizing solution or activation solution, and it may also be caused by pH value, temperature, formaldehyde, copper sulfate content of electroless copper plating solution is too low or content of complexing agent is too high.

To deal with this type of failure, first check whether sensitization solution, activation solution or colloidal palladium solution is normal. Use a small amount of newly prepared sensitizer and activation solution (or colloidal palladium solution). If roughened plastic parts can be deposited with a copper layer after being treated with newly prepared sensitizer and activation solution, it is proved that original sensitization solution or activation solution has failed, and these solutions should be adjusted or replaced.
If roughened light-colored plastic parts are treated with original sensitizing and activating solution, surface of part can become brown, indicating that sensitizing solution and activating solution have not failed, electroless copper plating solution should be checked.
When checking electroless copper plating solution, first check pH value and temperature of solution, adjust pH value of solution to about 12, control temperature at about 30℃, replenish an appropriate amount of formaldehyde before proceeding with trial plating. If copper layer cannot be deposited on parts, color (light color) or composition analysis of electroless copper plating solution should be used to determine whether copper sulfate content in plating solution is low. If content is low, an appropriate amount of copper sulfate main salt should be added. After such analysis and processing, phenomenon that parts cannot be plated with copper can be eliminated.

2. When electroless copper plating. Partially unplated copper layer on the surface of parts is different from partial unplated parts and completely unplated parts. Reason for this kind of failure is: Partially unplated parts on the surface may result in poor degreasing; poor roughening; insufficient sensitization or activation time; plastic parts themselves have stress or chemical copper plating solution composition imbalance, etc.

Failure of plating caused by incomplete degreasing of parts only occurs on a few parts and some parts of the parts, and not all parts have oil on the surface.
Therefore, this kind of failure phenomenon is rare, and good degreasing measures can be taken to eliminate it; if plating failure caused by stress of plastic itself usually appears on similar parts of parts, a small amount of parts can be heat treated to remove stress, then usual roughening, sensitization, activation and electroless copper plating can be used to check to eliminate this failure; chemical copper plating liquid composition imbalance is usually manifested as slow copper sinking speed.
If during electroless copper plating, surface of plastic part starts to sink within 5 minutes, it is not a problem of composition of copper plating solution; if there is no copper layer on the surface of plastic part for more than 15 minutes during electroless copper plating, it may be reason that pH value of electroless copper plating solution is too low, temperature is low, formaldehyde content is low,  copper sulfate content is low, or complexing agent content is too high. Analytical solution can be adjusted in proportion to eliminate this barrier.
In addition, content of chromic acid in roughening solution is insufficient, roughening temperature is too low or time is too short to cause roughening failure, resulting in partial surface of parts not being plated with copper.
plastic electroplating 
Different plastics need different coarsening temperature and time. Most domestic ABS plastics only need to be coarsened at 60℃~65℃ and time is 20min~30min. High coarsening temperature is easy to cause excessive coarsening and deformation of plastic parts.
However, temperature of 60℃~65℃ for Japanese electroplating grade ABS plastic is a bit too low, and it is easy to cause poor roughening of parts. Generally, it is better to roughen at 70℃~75℃. Of course, it also depends on whether parts are deformed. If parts are easily deformed, roughening temperature should be lower and time should be longer.
For some parts that are not easily deformed, roughening temperature can be higher, and time should be shorter. In some factories, temperature is controlled at 82℃~85℃ during roughening, and time is shortened to 2min~5min. In this way, roughened parts will light up quickly and have good brightness when electroless copper plating is performed on bright copper.
Therefore, during roughening, temperature and time of roughening solution should be mastered according to different plastic materials to prevent electroplating failure caused by poor roughening. During electroplating process of plastic signs in a factory, plastic parts sent by supporting factory are not regular injection molded parts, but parts that have been machined on injection molding plate and surface polished. Conventional injection molded parts electroplating process is used for electroplating.
It was found that appearance of part was normal before roughening, but after chemical roughening process, several yellow spots appeared on the surface of part, plating layer in this area became black during electroless copper plating, and there was a failure of plating after electroplating.
Although chemical roughening process has been adjusted many times, heat treatment time to eliminate internal stress has been extended, chemical reduction process has been strengthened, results have been minimal. After investigating production process of plastic blanks, it was learned that blanks of ABS injection molding plates were not completely cleaned by injection molding machine, and there were a small amount of black inclusions on injection molding plates.
Although there are no defects on the surface of parts after mechanical processing and polishing, due to large stress in this area during roughening process, it is easy to over-roughen and form fine cracks or looseness. In this way, in addition to reaction in activation process, a part of unreacted solution penetrates into cracks, which cannot be completely removed even though it is washed with water and reduced with formaldehyde.
As a result, electroless copper plating cannot proceed according to normal reaction process, and a black non-conductive Cu20 layer is formed in this area, resulting in occurrence of partial plating leakage. After factory switched to electroless nickel plating process for metallization of plastic parts, above-mentioned failures were avoided.
This is because reducing ability of reducing agent (sodium hypophosphite) in electroless nickel plating solution is far greater than formaldehyde in electroless copper plating solution, thereby eliminating this failure.
In addition, if time for sensitization and activation is not enough, there is often a phenomenon that plastic parts cannot be plated with copper. Sometimes plastic parts that have been sensitized and activated once do not form activated silver locally, so there will be a phenomenon that copper cannot be plated locally.
In order to eliminate this kind of failure, after the first sensitization and activation, second sensitization and activation treatment can be repeated after washing with water, even third and fourth sensitization and activation treatments can be performed to make roughening process. All parts of surface of plastic parts have catalytic silver atoms, which eliminates phenomenon that parts cannot be plated with copper.

3. Poor adhesion between coating and plastic. Poor adhesion of coating on plastic parts is a common failure phenomenon. General reasons for this failure are: incomplete degreasing on the surface of plastic part; improper plastic injection molding conditions; insufficient surface roughness or excessive roughening; oil in electroless copper plating solution will cause blistering and peeling of coating.

Incomplete degreasing of general parts is an accidental rare phenomenon. It can be avoided by strengthening degreasing measures before plating; poor adhesion of coating caused by improper injection molding conditions often occurs in same part of each part, and there will be many parts with this failure phenomenon.. Whether this is reason, you can take a small amount of parts, put them in oven to remove stress, then go through roughening, sensitization, activation and electroless copper plating, observe whether adhesion of coating is good after electroplating .
Poor adhesion of coating caused by insufficient roughening of part often appears on concave surface of part. If coating is peeled off, plastic surface of these parts is usually smooth, poor adhesion of coating caused by excessive roughening is more likely to appear on the tips and edges of parts. After coating is stripped, these parts show a rough appearance.
Way to check whether roughening is appropriate is to place roughened parts in the sun or dry them in an oven. If surface becomes white after drying, fine powdery substances appear on the tips and edges of parts. This powdery substance can be wiped off by hand, indicating that these parts have been excessively coarsened, coarsening temperature should be lowered or coarsening time should be reduced.
If a few parts of surface of dried part are white, most of surface is still natural color of plastic, it indicates that roughening is not sufficient, roughening temperature should be increased or roughing time should be increased.
What kind of roughening temperature and time should be used? It is best to take a small number of parts and test in different roughening temperatures and times in batches to find out that roughened parts can be hydrophilic and on the surface after drying. It is better to control roughening temperature and time under such temperature and time to show uniform whitening temperature and time when there is no powder.
Use of air stirring to improve crystalline structure of electroless copper plating layer, reduce copper powder, and prolong service life of copper plating solution has certain benefits. Mechanism of chemical copper plating relying on stirring to improve crystallization of plating layer and extend life of plating solution is to help quickly remove hydrogen generated by reaction in plating solution, which acts as second reducing agent in plating solution, but its presence will lead to excessive activity of local plating solution; in addition, it is necessary to strictly purify and stir air that passes people. Otherwise, if oil in air compressor is brought into copper plating solution, copper plating layer will be rough and loose, bonding force will be poor.
Because electroless copper plating solution is alkaline, if oil entering copper plating solution is saponified oil, a saponification reaction will occur to produce soap and glycerin. If non-saponified oil enters solution, it will be partially emulsified under long-term air stirring, resulting in foam in copper plating solution.
Therefore, when it is observed that there is foam in copper plating solution, copper layer is rough and loose at the same time, bonding force is not good, check whether air passing through copper plating solution is oily. If there is oil, it is necessary to strengthen air purification, in addition, copper plating solution must be treated with activated carbon to remove oil stains and eliminate failure of poor adhesion of coating.
Checking bonding force of coating of plastic parts can use cross-cut method, that is, use sharp edge spacing of lmm to draw 10 vertically and horizontally. Scratches must expose plastic matrix, then press and stick with specified adhesive tape. When adhesive tape is pulled apart, at least 90% of coating does not fall off or coating does not stick off at all to be qualified.
In addition, thermal cycle test can also be used to assess adhesion of coating of plastic parts. Generally, it is required to place plastic plated parts at -30℃ for 1 hour, then at room temperature for 1 hour, then at 70℃ for 1 hour, finally to room temperature for 1h as a cycle. Requirement of high bonding performance is that no blistering of coating after 4 cycles is qualified.

4. There are dark brown powders on copper plating layer. Reasons for this failure are: too much parts loading during electroless copper plating; excessive roughening; pH or temperature of copper plating solution is too high; content of copper salt is too high and content of complexing agent is too small; content of reducing agent is too much or copper plating solution is contaminated by activation solution.

When analyzing and dealing with such failures, first check whether electroless plating load of parts is too much. If load is really too much, reduce number of electroless copper plating parts to see if this kind of malfunction can be eliminated; Shorten chemical roughening time to check whether plastic parts are excessively roughened. If it is not for above reasons, check electroless copper plating solution.
Generally speaking, pollution of copper plating solution by activation solution is different from abnormal composition and operating conditions of electroless copper plating solution. The latter usually speeds up copper plating speed, speed is too fast, resulting in the formation of a dark brown powdery layer. Speed is not fast, it may be caused by pollution of activation solution.
From chemical copper plating solution composition and operating conditions abnormality caused such failures, it is found that pH value of chemical copper plating solution is too high, and such failures are prone to occur. Therefore, it is necessary to strictly control pH value of electroless copper plating solution (around 12) in production. Do not increase pH value of electroless copper plating solution in order to pursue a faster copper plating speed.
If pH value of electroless copper plating solution is controlled at about 12 and a dark brown powdery copper layer appears, it may be that electroless copper plating solution needs to be supplemented with a complexing agent, or plating solution can be appropriately diluted to control temperature of chemical copper plating solution below 30℃. These measures are conducive to troubleshooting this failure.

5. Black powder on electroless nickel plating layer is mainly caused by high temperature of electroless nickel plating solution or high pH value of plating solution, high content of nickel sulfate and sodium hypophosphite in electroless nickel plating solution.

These factors will cause reaction of electroless nickel plating solution to be violent, causing solution to decompose rapidly and produce solid nickel particles in the solution, inclusion of these particles becomes black powder in nickel plating layer.
Way to troubleshoot this type of fault is: first need to control pH value and temperature of plating solution, then analyze and adjust solution composition, and filter solid particles in electroless nickel plating solution.

6. Pockmarks of plastic coating (copper or nickel). Main sources of pits on the surface of plastic parts are: base material, pre-electroless plating process, copper plating and nickel plating. Small holes, small spots, inclusions, etc. on the base of plastic parts may cause pitting in coating; during roughening, there are small spots on the surface of part that are not wetted, activated to small spots without plating after electroless plating, then electroplating forms pits; and these pits exist in the form of pits.

Eliminating pitting produced in this link is to strengthen pre-plating inspection of base material and treatment of pre-electroless plating process. Plastic parts are also prone to pitting in electroplating and nickel plating processes after chemical plating. If phosphorus content is low or too high, anode will dissolve abnormally and easily produce cuprous oxide particles, which will cause pitting in copper plating layer.
In addition, imbalance of brightener in plating solution or decomposition products of brightener will cause pitting of coating, which must be strictly controlled to eliminate surface pitting of electroplated copper layer.

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