How to improve reflow soldering when founding PCB burst-3

13-08-2019

Copper is a non-polar substance in the metallic state, so many adhesives have minimal adhesion to copper foil. If the surface of the copper foil is not treated, it cannot be sufficiently adhered and heat-resistant even with an adhesive having excellent properties.

         In the early stage, the surface of the copper foil was browned by chemical treatment to form reddish-brown cuprous oxide (Cu2O) on the surface of the copper foil. When it is bonded to a resin laminated substrate sheet, although the adhesion at room temperature is increased, peeling occurs at around 200 °C. This is because Cu2O is unstable to heat and is peeled off between the heated and copper foil.

In the 1960s, researchers at Toshiba Corporation of Japan found that after treatment with a special chemical solution, the black velvet film (CuO) formed on the surface of the copper foil was finely crystallized and firmly adhered to the surface of the copper foil. The stability is also very good, which is the blackening process that is commonly used later.

         In the mid-1990s, Europe, the United States and other countries used a new type of multi-layered plate inner layer conductive pattern chemical oxidation of the new browning process, replacing the traditional blackening process, has been widely used in the industry.

2 Browning enhances adhesion mechanism

         The new browning process, the chemical reaction mechanism is:

         2Cu + H2SO4 + H2O2 + nR1 + nR2 → CuSO4 + 2H2O + Cu(R1 + R2) Figure 1.5 SEM image of substrate copper foil browning (×3000)

         In the browning tank, due to the micro-etching action of H2O2, the surface of the base copper is formed into an uneven microstructure, so that a bonding area equivalent to 6 to 7 times of the untreated smooth copper surface can be obtained. At the same time, a thin layer of an organic metal film bonded to the copper surface of the substrate by a chemical bond is deposited on the base copper, and the SEM image of the copper surface of the substrate is as shown in FIG. And after the adhesive enters the uneven portion, the mechanical meshing effect is also increased.

3 Factors affecting the browning effect

         The quality and effect of browning is determined by the refinement of the control of its process parameters, such as:

         (a) Choose a formula with advanced potions

b) Strengthen the monitoring of the bath components in the production process. (c) Browning (or black copper oxide) film thickness: Browning (or black copper oxide) The bonding strength, acid and alkali resistance, corona resistance and high temperature resistance of the film to PP are related to the structure and thickness of the film. . But it is not the thicker the bond strength is. (d) Browning layer contamination and process error: In a case of blasting quality, peeling off the site where the blast occurred, and found that the browning layer was contaminated, and the resin was completely separated from the contaminated browning layer.

The browning layer and the pp sheet which caused the contaminated portion were not effectively bonded after lamination, and the PCB board was foamed in the subsequent SMT assembly. After investigation, the high Tg material misuses the common material procedure for pressing and curing, which is also one of the reasons for the poor bonding force between the outermost copper foil and the pp sheet.


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