Three Main Reasons for Copper Rejection in PCB Factories
1.1 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 foil, red foil and ashing foil below 18um basically do not have batch copper rejection. When the customer’s circuit is designed to pass the etching line, if the copper foil specification is changed but the etching parameters remain unchanged, the copper foil stays in the etching solution for a long time. Because zinc is 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, causing the backing zinc layer of some thin circuits to be completely reacted and separated from the substrate. That is, the copper wire falls off. Another situation is that there is no problem with the PCB etching parameters, but the washing and drying after etching are not good, causing the copper wire to be surrounded by the etching solution left on the PCB surface. Dump copper is generally concentrated on thin lines, or during humid weather, similar defects will appear on the entire PCB. Strip the copper wire to see that the color of the contact surface with the base layer (the so-called roughened surface) has changed. The color of the copper foil is different from that of the normal copper foil. The original copper color of the bottom layer is seen, and the peeling strength of the copper foil at the thick circuit is also normal.
1.2 A local collision occurs in the PCB process, and the copper wire is separated from the substrate by external mechanical force. This defect is manifested as poor positioning or orientation, and there will be obvious twists in the dropped copper wire, or scratches/impact marks in the same direction. Peel off the bad copper wire and look at the rough surface of the copper foil. It can be seen 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.
1.3 The PCB circuit design is unreasonable. Designing too thin circuits with thick copper foil will also cause excessive circuit etching and copper rejection.
Under normal circumstances, as long as the laminate is hot-pressed for more than 30 minutes in the high temperature section, the copper foil and the prepreg are basically completely combined, so the pressing generally does 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 PP is contaminated, or the rough surface of copper foil is damaged, it will also lead to insufficient bonding force between copper foil and substrate after lamination, resulting in positioning (only for large boards and language) or sporadic copper wires fall off, but there is no abnormality in the peeling strength of copper foil near the off-wire.
3.1 It is mentioned above that ordinary electrolytic copper foils are galvanized or copper-plated wool foil products. If the wool foil is produced with an abnormal peak value, or when galvanized/copper-plated, the coating dendrites are not good, causing the peel strength of copper foil is not enough. After the poor foil pressed sheet is made into a PCB, when it is inserted in an electronic factory, the copper wire will fall off under the impact of external force. This kind of poor copper throwing will not cause obvious side etching after peeling the copper wire to see 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.
3.2 Poor adaptability between copper foil and resin: Some laminates with special properties currently used, such as HTg sheets, are generally PN resins as curing agents due to different resin systems. The resin molecular chain structure is simple, and when curing The degree of crosslinking is low, and it is necessary to use copper foil with special peaks to match it. When producing laminates, the use of copper foil does not match the resin system, resulting in insufficient peel strength of the metal-clad sheet metal foil, and poor copper wire falling off when inserting.
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