Cause Analysis of PCB Thin Hole Without Copper
The problem of shallow copper-free is very easy to occur during the PCB manufacturing process. Some customers' misjudgments about copper-free of this type of hole are as follows:
1. The tin-plating agent has poor ability of deep plating (position) and bad calling tin;
2. PTH is abnormal, and copper is not sinking in the hole;
3. Poor copper plating has poor deep plating ability.
In actual production, it is not uncommon for tapered holes to be free of copper. The reason is that there is no plating layer on the conductive substrate (plate copper or thick copper layer) that hinders the deposition of electroplated copper. The generation and prevention of this plating resist are analyzed below.
During the development of the next step of the circuit of copper or heavy copper, the uncross-linked polymerized ink on the PCB surface is dissolved in the developer solution, and the developer solution containing the ink polymer is sprayed again on the PCB surface and holes through the circulation pump. At this time, if the subsequent pressure water washing (water-containing washing water quality) is not enough to clean the residue of the ink polymer contained in the surface of the PCB and the hole, the PCB proofing then the remaining ink polymer compound will be de-adhered on the hole wall to form A thin layer of the plating resist layer, the worse the cleaning effect is, the more it is to the center of the hole, the greater the probability of the occurrence of the plating resist layer, especially the small holes. (Multi-stage water washing in the developing section is just a process of continuously diluting the residue, the purpose is to dilute the residue as much as possible).
After understanding that the polymer anti-adhesive coating is the culprit causing the thinning of the copper layer in the hole, the focus of the problem is to ensure the cleaning effect in the hole to remove the anti-adhesive plating. The right medicine can cure the root cause.
In addition, the premise of dealing with realistic problems is that we must face up to and respect the existing production conditions of customers, such as: lines and solder masks, dry film and wet film shared developing machines, and water washing flow is subject to environmental restrictions.
Some customers had hoped that increasing the amount of micro-etching in the pre-treatment of the picture could remove the resist coating in the hole, but unfortunately it was not helpful, but the excessive micro-etching caused the hole to be free of copper. The correct solution should be to strengthen the maintenance of the developing dry process, and at the same time, select the acid degreasing agent with good degreasing effect before the electric treatment.
EC-51 acid degreasing agent can cooperate with customers to solve the problem of copper pores that are gradually thinning from the hole opening to the center of the hole. The correct use of EC-51 acid degreasing agent requires the following:
1. EC-51 water washing requirements are slightly stricter, and sufficient water washing is required. Poor cleaning with the wetting agent contained in it may cause more foam in copper and nickel cylinders.
2. EC-51 is designed for wet film. If wet or black oil plate is used, if the hole is not plated with nickel or copper, it can be resolved after treatment with EC-51. For thin-line dry films, the amount of cylinder opening should be appropriately reduced, and the content of EC-51 should be controlled to 4% to prevent excessive levels of degreasing agent from attacking the edges of the dry film to cause canine tooth-like coating. In addition, EC-51 gradually thins the dry film Hole-free copper effect is also good.
3. Winter is a high-incidence period for such problems (because of low temperature and poor washability). The most effective way to improve the effect of degreasing is to raise the temperature (the increase in concentration does not contribute much, and the pressure of washing is increased). The temperature is generally controlled 30- 35 degrees, too low temperature is not conducive to ensuring the degreasing effect; too high temperature is prone to oil degreasers attack the ink and lead to penetration. The PCB is manufactured in a manual line, and it should also be equipped with a manual swing and a filter to ensure that the liquid in the hole penetrates. If the customer's production conditions are bad, the cylinder change cycle of EC-51 should be shortened to 15-20 square feet / liter.