PCB Technical - Understand the causes of PCB dumping

When the copper wire of the PCB falls off, each PCB brand will say that it is a laminate problem and require its production plants to bear bad losses. According to years of experience in handling customer complaints, the common reasons for PCB dumping are as follows:

1. PCB factory process factors:

1. Excessive etching of copper foil.

The electrolytic copper foils used in the market are generally single-sided galvanized (commonly known as ashing foil) and single-sided copper-plated (commonly known as red foil). Common copper foils are generally galvanized copper foil over 70um, red foil and 18um. The following ashing foil has basically no batch copper rejection. When the circuit design is better than the etching line, if the copper foil specifications are changed without changing the etching parameters, this will cause the copper foil to stay in the etching solution for too long.

Because zinc is originally an active metal, when the copper wire on the PCB is immersed in the etching solution for a long time, it will cause excessive side corrosion of the circuit, causing some thin circuit backing zinc layer to be completely reacted and separated from the substrate, namely The copper wire falls off.

Another situation is that there is no problem with the PCB etching parameters, but the washing and drying are not good after etching, causing the copper wire to be surrounded by the residual etching solution on the PCB surface. If it is not processed for a long time, it will also cause excessive side etching of the copper wire. copper.

This situation is generally concentrated on thin lines, or when the weather is humid, similar defects will appear on the entire PCB. Strip the copper wire to see that the color of its contact surface with the base layer (the so-called roughened surface) has changed, which is different from that of normal copper. The color of the foil is different, the original copper color of the bottom layer is seen, and the peeling strength of the copper foil at the thick line is also normal.

2. In the PCB production process, a collision occurs locally, and the copper wire is separated from the substrate by mechanical external force.

This bad performance has a problem with the positioning, and the copper wire will be obviously twisted, or scratches or impact marks in the same direction. If you peel off the copper wire at the defective part and look at the rough surface of the copper foil, you can see that the color of the rough surface of the copper foil is normal, there will be no side erosion, and the peeling strength of the copper foil is normal.

3. The PCB circuit design is unreasonable.

Using thick copper foil to design a circuit that is too thin will also cause excessive etching of the circuit and dump copper.

2. Reasons for laminate manufacturing process:

Under normal circumstances, the copper foil and the prepreg will be basically completely combined as long as the high temperature section of the laminate is hot pressed for more than 30 minutes, so the pressing will generally not affect the bonding force of the copper foil and the substrate in the laminate. However, in the process of stacking and stacking laminates, if PP contamination or copper foil rough surface damage, it will also cause insufficient bonding force between the copper foil and the substrate after lamination, resulting in positioning deviation (only for large plates) ) Or sporadic copper wires fall off, but the peeling strength of the copper foil near the off wires will not be abnormal.

3. Reasons for laminate raw materials:

1. As mentioned above, ordinary electrolytic copper foils are all products that have been galvanized or copper-plated on wool. If the peak value of the wool foil is abnormal during production, or when galvanizing/copper plating, the plating crystal branches are bad, which will cause the copper foil itself. The peeling strength is not enough. After the bad foil pressed sheet material is made into a PCB, the copper wire will fall off when it is impacted by an external force when it is plugged in in the electronics factory. This kind of poor copper rejection will not have obvious side corrosion when peeling the copper wire to see the rough surface of the copper foil (that is, the contact surface with the substrate), but the peeling strength of the whole copper foil will be very poor.

2. Poor adaptability of copper foil and resin: some PCB laminates with special properties, such as HTG sheets, are used now, because of the different resin systems, the curing agent used is generally PN resin, and the resin molecular chain structure is simple and curing When the degree of cross-linking is low, it is necessary to use a copper foil with a special peak to match it. The copper foil used in the production of laminates does not match the resin system, resulting in insufficient peel strength of the sheet metal-clad metal foil, and poor copper wire shedding when insertng.