Analysis on the Reason of No Copper in Thinning Hole

Hole metallization is the most important process in the PCB manufacturing process. This article talks about a personal understanding and understanding of the copper-free appearance, causes and solutions of a thinning type of hole.

The thinning type of hole has no copper in common, that is, the copper layer in the hole gradually thins from the hole to the center of the hole until the copper layer disappears.

Some customers’ misjudgments about this type of hole without copper are as follows:

1. The tin light agent has poor deep plating (positioning) ability and causing poor electrical tin.
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2. The PTH is abnormal, and copper is not sunk in the hole.
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3. The deep plating ability of copper plating is poor.

In actual production, it is not uncommon to see tapered holes without copper. The reason for this is nothing more than the existence of a resist layer on the conductive substrate (plate-electro-copper or heavy-duty copper layer) that hinders the deposition of electroplated copper. The following is an analysis of the generation and prevention of this kind of resist.
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In the process of circuit development in the next process of PCB electroplating-copper or heavy copper, the uncrosslinked and polymerized ink on the PCB surface is dissolved in the developer, and the developer containing the ink polymer is sprayed again to the PCB surface and holes by the circulating pump. At this time, if the subsequent pressure washing (with washing water) is not enough to wash the residues containing ink polymers on the PCB surface and holes, the remaining ink polymer compounds will stick to the hole wall to form a layer thinner coating resists. The worse the cleaning effect is, and the more likely the resist coating is to appear, especially for small holes. The multi-stage water wash in the development section is just a process of continuously diluting the residue, in order to dilute the residue as much as possible.

After understanding that the polymer anti-stick 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-stick coating. The right medicine can cure the root cause.

In addition, the premise of dealing with practical problems is to face up to and respect the existing production conditions of customers, such as: lines and solder masks, shared development machines for dry film and wet film, and environmental protection restrictions on water washing flow.

Some customers have hoped that increasing the amount of micro-etching in the pre-treatment of the drawing can remove the resist coating in the hole, but unfortunately it does not help. The correct solution should be to strengthen the maintenance of the development and drying process, and at the same time, choose an acid degreaser with excellent degreasing effect in the pre-treatment of the graph.

EC-51 acid degreaser can cooperate with customers to solve the phenomenon that the hole copper gradually thins from the orifice to the center of the hole without copper. The correct use of EC-51 acid degreaser should pay attention to the following matters:

1. EC-51 has slightly stricter water washing requirements, and requires sufficient water washing, because the wetting agent contained in it may cause more foam in copper and nickel tanks.

2. EC-51 is specially designed for wet film, use wet film or black oil board, if the hole cannot be plated with nickel or copper, it can be solved after treatment with EC-51. For fine line spacing dry film, the cylinder opening amount should be appropriately reduced, and the EC-51 content should be controlled to 4% to prevent excessive degreaser content from attacking the dry film line edge and causing canine-shaped coating. In addition, EC-51 is gradually thinning for dry film The hole-free copper effect is also good.

3. Winter is a high incidence period for such problems (due to low temperature and poor water washability), the most effective way to improve the oil removal effect is to increase the temperature (the increase in concentration does not contribute much, and the water washing pressure will be increased), and the temperature is generally controlled at 30- 35 degrees, too low temperature is not conducive to ensuring the degreasing effect. Too high temperature is prone to the degreaser attacking the ink and causing bleeding. In the manual line, manual swing and filter should be added to ensure the penetration of the liquid in the hole. If the customer’s production conditions are bad, the cylinder replacement cycle of EC-51 should be shortened to 15-20 square feet/liter.