The copper wire of PCB falls off (also known as copper throwing), and PCB factories all say that it is the problem of laminate, which requires its production factory to bear the bad losses. According to customer complaint handling experience, there are several common reasons for copper rejection in PCB factory:
I. Process Factors of PCB Factory
1. The copper foil is etched excessively. Electrolytic copper foil used in the market is generally single-sided galvanized (commonly known as ashed foil) and single-sided copper plated (commonly known as reddening foil). The common copper throwing is generally galvanized copper foil above 70μ m, and the reddening foil and the ashed foil below 18μ m have basically never seen batch copper throwing.
When the customer’s circuit design is better than the etching line, if the copper foil specification is changed but the etching parameters are unchanged, the residence time of the copper foil in the etching solution is too long. Because zinc is originally an active metal, when the copper wire on the PCB of LED advertising screen is soaked in etching solution for a long time, it will definitely lead to excessive side etching of the circuit, resulting in some thin circuit backing zinc layers being completely reacted off and separated from the substrate, that is, the copper wire falls off.
In another case, there is no problem with the etching parameters of PCB, but poor washing and drying after etching cause the copper wire to be surrounded by the residual etching solution on the surface of PCB. If it is left untreated for a long time, it will also lead to excessive side etching of copper wire and copper throwing. Generally, this kind of situation is concentrated on thin lines, or when the weather is wet, similar defects will appear on the whole PCB. When the copper wire is peeled off, the color of its contact surface with the base layer (the so-called roughened surface) has 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 thick line is also normal.
2. Local collision occurs in the PCB process of LED advertising screen, and the copper wire is separated from the base material by external mechanical force. This defect is characterized by poor positioning or orientation, and the detached copper wire will be obviously twisted or scratched/hit in the same direction. If you look at the matte surface of the copper foil by stripping the copper wire at the defective part, you can see that the matte surface of the copper foil is normal in color, and there will be no side erosion, and the peeling strength of the copper foil is normal.
3. The circuit design of LED advertising screen circuit board is unreasonable. If the circuit is too thin with thick copper foil, it will also cause excessive etching of the circuit and throw away copper.
Second, the laminate process reasons
Under normal circumstances, as long as the laminate is hot-pressed for more than 30min, the bonding between the copper foil and the prepreg is basically complete, so the bonding force between the copper foil and the substrate in the laminate is generally not affected by pressing. However, in the process of laminating and stacking laminates, if PP is polluted or the matte surface of the copper foil is damaged, the bonding force between the laminated copper foil and the base material will be insufficient, resulting in positioning (only for large boards) or sporadic copper wires falling off. However, there will be no abnormality when measuring the peeling strength of the copper foil near the off-line.
III. Reasons of Laminate Raw Materials
1. It is mentioned on the LED advertising screen that ordinary electrolytic copper foils are products treated with wool foil galvanizing or copper plating. If the peak value of wool foil is abnormal during production, or the crystal branches of the coating are poor during galvanizing/copper plating, the peeling strength of the copper foil itself is not enough. When the defective foil is pressed into PCB and inserted into the 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 erosion after stripping the copper wire (that is, the contact surface with the base material), but the peeling strength of the whole copper foil will be very poor.
2. Poor adaptability between the copper foil of LED advertising screen and resin: For some laminates with special properties, such as HTg sheet, due to different resin systems, the curing agent used is generally PN resin. The molecular chain structure of the resin is simple, and the degree of crosslinking is low during curing, so it is necessary to use copper foil with special peak value to match it. When the copper foil used in the production of laminate is not matched with the resin system, the peeling strength of the metal foil coated on the sheet is not enough, and the copper wire falls off badly when inserting.