1. The copper foil is over-etched. 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 reddish foil). The common copper rejection is generally galvanized copper above 70um Foils, red foils and ashing foils below 18um have never seen batches of PCB copper rejection.
The customer's circuit has planned the timing of the etching line. If the copper foil standard is changed but the etching parameters remain unchanged, the copper foil will 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 inevitably lead to excessive side etching of the circuit, and the backing zinc layer of some thin circuits will be completely reacted, and The substrate is detached, that is, the copper wire falls.
Another situation is that there is no problem with the etching parameters of the PCB, but after etching, washing and drying are not good, and the constituent copper wires are also surrounded by the etching solution left on the surface of the PCB instrument. The copper wire is over-etched and the copper is thrown away.
This condition is generally concentrated on thin lines, or during periods of humid weather, similar defects will appear on the entire PCB.
Peel off the copper wire to see that the color of its contact surface with the bottom layer (the so-called roughened surface) has been changed, which is different from the color of the normal copper foil. What you see is the original copper color of the bottom layer, and the peeling strength of the copper foil at the rough circuit is also normal.
2. Some bumps occur in the PCB process, and the copper wire is separated from the base material by external mechanical force. This defect is manifested by poor positioning or orientation, and the falling copper wire will have obvious distortions, or scratches/bump marks in the same direction.
Peel off the bad copper wire 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 bad side corrosion, and the peeling strength of the copper foil is normal.
3. The layout of the PCB is unreasonable. Using thick copper foil to plan too thin circuits will also cause excessive circuit etching and copper rejection.
Under normal conditions, the laminate only needs to be hot-pressed for more than 30 minutes in the high temperature section, and the copper foil and the prepreg will be completely combined, so the lamination will generally not affect the bonding force between the copper foil and the substrate in the laminate.
However, in the process of stacking and stacking laminates, if the PCB is contaminated, or the rough surface of the copper foil is damaged, it will also lead to the lack of bonding force between the copper foil and the substrate after lamination, which constitutes positioning (only for For large boards) or scattered copper wires fall, but the peeling strength of copper foil near the test strip will not be abnormal.
1. It is mentioned above that ordinary electrolytic copper foils are galvanized or copper-plated wool foil products.
If the peak value of wool foil is abnormal during production, or when galvanized/copper-plated, the dendrites of the coating layer are not good, and the peel strength of the copper foil itself is not good. The copper wire will fall when it is impacted by external force.
This kind of poor copper rejection will not cause obvious side etching after peeling the copper wire to see that the rough surface of the copper foil (that is, the surface in contact with the substrate), but the peeling strength of the copper foil on the whole surface will be very poor.
2. Poor adaptability between copper foil and resin: some special functional laminates currently used, such as HTg sheet, because of the different resin systems, the curing agent used is generally PN resin, and the molecular chain structure of the resin is simple. The degree of cross-linking is low, and copper foil with a special peak must be used to match it.
When the laminate is produced, the use of copper foil does not match the resin system, and the peeling strength of the metal foil cladding of the PCB is not good, and the copper wire will also fail to fall during the plug-in.