PCB immersion silver board chemical process concept

2019-08-24 20:34:23

In recent years, chemical immersion silver plates have been widely used in the final surface treatment process of PCBs due to their advantages of simple operation, reasonable price and excellent performance. After the chemical immersion silver plate, the copper hole is a key quality monitoring index. If there are too many voids in the silver under the silver layer, the micro-cavities in the subsequent solder joints will be too dense, which will lead to the vibration failure and temperature-induced fatigue failure of the solder joints. problem.

There is a dense void at the interface of the silver plate of the PCB. The copper layer at the edge of the cavity is easily oxidized to contain copper oxide. During the soldering process, the silver layer dissolves and diffuses into the solder, and the flux spreads on the surface of the pad into the copper cavity. On the one hand, the flux extrudes the gas in the cavity outside the hole, and is wrapped by the tin material. On the other hand, it chemically reacts with the copper wall oxide to reduce the copper-containing oxide and simultaneously generate a small amount of small molecules of gas. The gas and the reaction-generating gas continuously move upward in the molten tin due to buoyancy, and finally enriched in the surface of the IMC surface. After the solder joint is cooled, a neat microcavity is formed in the IMC layer, resulting in solder joints. The bonding force is weakened, and the joint breakage failure occurs in the subsequent stress screening process such as temperature shock and vibration test.

Cavity production mechanism conjecture

At present, there are few studies on the copper voids under the immersion silver plate. The earliest speculation on the mechanism of the copper under the silver layer originates from Intel. The Intel report speculates that the galvanic effect in the process of sinking silver may cause voids in the copper layer, but the mechanism is not specifically described. Lu Zhiqiang et al. combined with the reaction process of Shenyin production, speculating that the priming cell effect causes the void mechanism of the silver under copper layer as follows: in the process of immersion silver reaction, some areas (such as solder resist residue, contaminant and copper surface junction or rough) Higher degree, etc.) The exchange of syrup is slower. Here, the supply of silver ions is insufficient. The copper surface cannot deposit the silver layer on the PCB in time. After the silver is deposited in other areas, it will form two levels of electrodes with the copper surface. The immersion silver circuit forms a primary battery circuit. Under the reaction of the primary battery, the copper layer at the place continuously loses electrons to form copper ions, which are corroded and finally form a cavity.

In addition, too much local silver precipitation reaction rate may form copper voids. The rate of partial displacement reaction is too fast, and the silver ion consumption in the solution is too late to be replenished, resulting in a copper etch rate higher than the silver deposition rate. The deposited silver layer cannot completely cover the copper surface, and the uncovered portion also forms in the continuous replacement reaction. Empty,

There is also a possibility that when the copper surface has pits or the deposition rate of the silver solution is too fast, a trace amount of the syrup remains in the silver layer during the silver immersion process, and the copper surface is continuously occluded to cause voids in the subsequent placement.

Influencing factor

According to the above three kinds of cavity formation mechanism, combined with the actual chemical sinking silver production process, the influence of relevant production factors on the sinking silver void is analyzed and inferred.

First, consider the factors that may affect the immersion silver process of the galvanic cell effect. The degreasing process will affect the copper surface removal effect. Therefore, the cleanliness of the copper surface after degreasing will cause the galvanic effect to differ in the degree of copper surface biting, and ultimately affect the copper void. In the subsequent micro-etching process, different types of micro-etching agents will have significant differences in copper surface erosion, and the galvanic cell effect will be different. If the copper surface is not cleaned after micro-etching, there is residual residue in the micro-etching solution. The copper surface is more bitten by the syrup, and the probability of forming a void due to the galvanic effect in the subsequent silver sinking process increases. In addition, at different production temperatures, the reaction rate and severity of the sinking silver syrup are different, so the temperature of the immersion silver bath may also affect the copper void. In addition, considering the different types of copper foil, the crystallization mode is different, and the copper atoms on the surface are loosely arranged. In the process of sinking silver, the intensity of the replacement reaction with the silver syrup will be different, and the reaction is severe. It may be bitten out of the void, so it is also an influence point for the type of copper foil.

Therefore, in the actual power production process, there are many possible influencing factors affecting the cavity of the silver plate. Combined with the relevant DOE experiment conclusions of the study on the key influencing factors of the interface void under the silver plate, the microetching agent type and The different effects are one of the prominent factors affecting the copper void at the interface after the sinking of silver. In actual PCB production applications, it is important to pay attention.

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