Introduction

In the manufacturing process of electrical connectors, the shell of some products is more difficult to machine molding due to its complex shape, and most of them are now die-casting molding. However, due to the die-casting aluminum shell matrix material density is low, by chemical nickel plating after plating of the layer of high porosity, will bring some impact on the corrosion resistance of the product plating.

Especially some larger die-casting aluminum shell, due to the die-casting machine tonnage is not enough, die-casting processing out of the shell surface local pinhole phenomenon, chemical nickel plating soon after, in the plating surface near the pinhole there will be a water stains like a dark spot, when serious, but also moldy and discoloration phenomenon, these phenomena seriously affect the normal delivery of the product.

In recent years, through the FJ72599 curved shell chemical nickel plating process research and improvement (see Figure 1), summed up in the die casting before plating, plating and post-processing to increase the process of solving the die casting aluminum shell of the above quality problems, and achieved certain results.

Figure 1 Comparison between photos of FJ72599 die-cast
aluminum electronic connector before (left) and after
(right) electroless nickel plating

II. Methods to improve the quality of die-casting aluminum shell chemical nickel plating layer

1.Pre-plating treatment

a. Electrical connector aluminum shell processing methods

In the electrical connector aluminum shell processing methods, there are cold extrusion processing (the use of molds will be deformed aluminum alloy direct extrusion molding), turning processing (using a lathe will be made of hard aluminum alloy bar turning into a shell) and die-casting molding 3 kinds of ways. Compared with turning and cold extrusion, die-casting is more efficient and suitable for processing aluminum shells with relatively complex shapes, such as tail accessories.

However, due to the die-casting processing of aluminum shell only in the substrate surface has a thin layer of dense layer, after the plating process of alkaline etching and acid treatment, the majority of this layer of dense layer is destroyed, the substrate pores are exposed a lot, ultimately leading to a large number of pores in the electroless nickel layer, the plating layer after the salt spray test appears to be obvious corrosion phenomenon (see Figure 2).

Figure 2 Photos of salt spray test for the die-cast
aluminum electronic connector with traditional electroless
nickel plating process

b. Shot blasting

In order to reduce and ultimately avoid this phenomenon, has used mechanical polishing, bowl type vibration polishing machine to add corundum abrasive vibration polishing and the use of steel shot blasting method for FJ72599 curved shell for different ways of pre-plating, and finally through the effect of the salt spray test results in the plating of the increase in shot blasting process program. The results of the salt spray test of the plating layers obtained by the new and old processes are shown in Fig. 3.

Figure 3 Comparison between the salt spray test results
of coatings gained from new and old processes

As can be seen in Fig. 3, the salt spray resistance of the shell plating of the shot peened shells is greatly enhanced due to the increased densification of the substrate compared to that of the shells treated before conventional plating.

c. Mechanical polishing

Plating before the use of mechanical polishing methods (can not be used chemical polishing, so as not to shell surface of the only dense layer destroyed) can improve the die-casting aluminum shell substrate surface finish. However, it is worth noting that in the use of mechanical polishing to have a certain degree of rigidity of the nylon polishing wheel rather than use cloth polishing wheel, although the former polishing efficiency and polishing the surface of the substrate after the brightness is lower than the latter, but it can be found in a timely manner the substrate there are defects (the surface of the sand, layer) of the plating, and plating plating after the appearance of the effect of the plating layer is much better (as shown in Figure 4).

Figure 4 Influence of different polishing ways on
appearances of deposit
Left－with nylon polishing wheel, right－with rag wheel

Figure 4 illustrates that the surface finish of the aluminum housing after plating is more consistent when it is polished with a nylon polishing wheel before chemical nickel plating (sample on the left). In contrast, polishing with a cloth wheel results in differences in surface finish due to the exposure of substrate defects (pores, pits) after plating.

d.Closure after plating

In order to further improve the salt spray resistance of the plating after die-casting aluminum shell chemical nickel plating, and also in order to avoid the shell after a short period of time to produce the dark spot of the water trace, the chromate solution chemical passivation after plating, after plating immersion in the metal protective agent and after plating coated with liquid paraffin to close the pores of the plating layer. 3 kinds of sealing method effect comparison is shown in Table 1.

Closure method	Salt spray resistance	Waterproof trace effect	Other impacts
Chromate passivation	Some improvement	obvious	Reduced solderability of plating
Dipped metal protectants	mark improvement	obvious	Slightly reduced solderability and brightness of the plating.
Liquid paraffin coated	Some improvement	obvious	Reduced solderability of the plating, contact resistance is significantly affected

Die-casting aluminum shell chemical nickel plating process flow

① Preparation of die-cast machined aluminum alloy housing shells.
② Perform shot peening (or mechanical polishing after shot peening).
③ Complete all necessary machining sequences.
④ Perform ultrasonic degreasing and cleaning of the shell with trichloroethylene.
⑤ Dry the shell and then promptly encapsulate it in a plastic bag.
⑥ Perform pre-plating inspection.
⑦ Separate and block the shells from the plated parts with aluminum wire hangers.
⑧ Perform chemical degreasing.
⑨ Carry out two times of running water cleaning.
⑩ Perform alkaline film removal.
⑪ Perform two more rinse cycles.
⑫ Perform a zinc dip using a nickel-containing zinc dip (nickel-containing zinc dip is recommended).
⑬ Dezincification of the zinc dip using nitric acid (HNO3) at a concentration of 70%.
⑭ Two further washing cycles in running water are carried out.
⑮ Two more zinc dips with nickel-containing zinc dipping solution.
⑯ Two washing cycles in running water.
⑰ Pure water dipping.
⑱ Low-temperature, low-phosphorus electroless nickel priming.
⑲ Performs two rinses in running water.
⑳ Perform pure water rinsing.
㉑ High-phosphorus nickel plating with thicker nickel layer.
㉒ Two times of running water cleaning.
㉓ Perform post-plating chemical passivation treatment.
㉔ Perform two running water rinses.
㉕ Hot water immersion cleaning.
㉖ Hot air drying or centrifugal drying.
㉗ Heat treatment at high temperature for 1 hour (stress relief of the plated layer).
㉘ Perform post-plating inspection.
㉙ Dipping and coating with metal protectors or liquid paraffin.
㉚ Drying again and drying with liquid paraffin using a clean fine cloth.

IV. Conclusion

There are many ways to improve the corrosion resistance of die-casting aluminum shell chemical nickel plating layer, at present, most of the automotive industry uses in chemical nickel plating before sealing the hole with a thicker layer of electroplated copper, and then mechanical polishing process; most of the electronics industry is used in the chemical nickel plating and then coated with a composite layer of polytetrafluoroethylene process.

These methods for the manufacturing process of electrical connector shells, the effect is good. However, from the production efficiency and production cost considerations are not as simple and easy as the method described in this paper. However, when using this process, the following issues need to be noted:

(1) For the appearance of higher requirements of the shell and small shell, in the use of shot blasting process, can be used to spray glass shot or ceramic shot to replace the steel shot, but also in the shot blasting and then use the mechanical polishing method to reduce the shell surface crater formation.

(2) In the use of post-plating closure method, the shell must be clean and thorough cleaning after plating, the moisture in the pores must be dried. In addition to the use of chromate passivation is carried out before drying (if the post-plating heat treatment temperature ≥ 180 ° C, the process is suitable to be placed in the heat treatment), the remaining two methods are carried out after drying, and after dip coating must again dry the shell so as not to leave traces on the surface of the plating.