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Causes for the copper wire of the impedance PCB circuit board to fall off

Date:
2020/05/13 16:40
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The copper wire of the PCB has fallen off (also commonly referred to as copper rejection). The PCB factories all say that it is a laminate problem, requiring their production plants to bear bad losses. According to years of experience in handling customer complaints, the common reasons for PCB factories to dump copper are as follows:
1. Factors of PCB factory process:
1. The copper foil is over-etched. The electrolytic copper foil used in the market is generally single-sided galvanized (commonly known as ashing foil) and single-sided copper (commonly known as red foil). The common copper throwing is generally galvanized copper above 70um Foil, reddish foil and ashed foil below 18um basically haven't appeared batch dump copper.
When the customer circuit is designed over the etching line, if the copper foil specifications are changed and the etching parameters are not changed, the residence time of the copper foil in the etching solution is too long. 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 cause excessive side corrosion of the circuit, causing some thin circuit backing zinc layers to be completely reacted and detached 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 water washing after etching and poor drying cause the copper wire to be surrounded by the remaining etching solution on the surface of the PCB. Throw copper. This situation is generally manifested as being concentrated on thin lines, or during periods of 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. Unlike the normal copper foil color, the original copper color on the bottom layer is seen, and the copper foil peel strength at the thick line is also normal.
2. A partial collision occurred in the PCB process, and the copper wire was separated from the substrate by external mechanical force. This bad performance is manifested as poor positioning or directionality, and there will be obvious distortion of the fallen copper wire, or scratches / impact marks in the same direction. Look at the copper foil matte surface of the copper wire in the bad peeling area. You can see that the color of the copper foil matte surface is normal, there will be no side corrosion, and the copper foil peeling strength is normal.
3. PCB circuit design is unreasonable. Designing too thin circuits with thick copper foil will also cause excessive etching of the circuit and throw away copper.
Second, the reasons for the laminate process:
Under normal circumstances, as long as the laminate is hot pressed at a high temperature for more than 30 minutes, the copper foil and the prepreg are basically completely combined, so the pressure bonding generally does not affect the bonding force of the copper foil and the substrate in the laminate. However, in the process of stacking and stacking of laminates, if PP pollution or damage of the copper foil rough surface will also result in insufficient bonding force between the copper foil and the substrate after lamination, causing positioning (only for large boards) Words) or sporadic copper wire, but there is no abnormality in the peel strength of the copper foil near the measured wire.
Third, the reasons for the raw materials of the laminate:
1. It is mentioned above that ordinary electrolytic copper foils are all galvanized or copper-plated products of wool foil. If the peak value of wool foil production is abnormal, or when zinc / copper plating, the plating layer is poor, resulting in copper foil itself. The peel strength is not enough. When the bad foil is pressed into a PCB and is inserted into an electronic factory, the copper wire will fall off when it is impacted by external force. This kind of copper flicking will peel off the copper wire to see that the copper foil matte surface (that is, the contact surface with the substrate) will not have obvious side corrosion, but the peel strength of the entire copper foil will be very poor.
2. The adaptability of copper foil and resin is poor: some of the special performance laminates currently used, such as HTg sheet, because the resin system is different, the curing agent used is generally PN resin, the resin molecular chain structure is simple, when curing The degree of cross-linking is low, and it is necessary to use special peak copper foil to match it. When producing laminates, the use of copper foil does not match the resin system, resulting in insufficient stripping strength of the metal foil on the sheet material, and the copper wire will also fall off wh
en inserting.

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