## 3 Test Details
### Some Main Modulators of TH FM on Electrical Contact  
#### 3.2.1 Main Modulators  
Based on the TH FM, several main modulators that are believed to have great impacts on TH performance, and can be implemented through relatively non-complicated design applications, were identified. They were:  
1. Enabling load: The theory is that the possibility of contacting points penetrating through the films/the foreign materials is a function of contact force. Increasing contact force increases the possibility of forming air tight contacting points. Thus, the solution was to increase the enabling load.
2. Surface finish thickness: The theory is that increasing the IG plating thickness helps reduce the number of the pores, and thus mitigate the formation of nickel oxidation in order to improve the resistance to TH. The solution was to increase the Au thickness.  
Figure 13: Schematic of the TH failure mechanism in electrolens nickel and immersion gold plating (ENIG) surface finish.  
Contacting area: The theory is that increasing contacting area helps improve the contacting efficiency, since larger contacting areas may provide more space for more gas tight contacting points. Note: The larger the contacting area, the smaller the contact pressure. Thus, in order for the contacting area to play a positive role, a minimum contact force was required. The solution was to increase the contacting area with the minimum contact force requirement being met.  
The first proposed solution of increasing the enabling load and the second proposed solution of increasing Au plating thickness were more straightforward, but the third one of increasing contacting area was tricky. It had to do with the detailed design of a contact tip. Additionally, the nominal contacting area was not equal to the actual contacting area since not all points within the nominal area were gas tight. It has been a challenge to investigate these gas tight points. There are some papers on this topic. The single-spot first-order model for constriction resistance established by [PERSON] [4] has received international recognition. On the basis of this model, many researchers have studied the influence of many factors such as multi-contact spot, temperature, arc, film, time, etc. A variety of complex theoretical models of contact resistance have been established. [PERSON] simulated the contact performance by considering the combined effects of surface degradation and interface motion on the change of contact resistance, and proposed a third level multi-spot constriction model [5; 6]. [PERSON] and [PERSON] studied the effect of normal forces on fitting corrosion in tin-coated electrical contacts, and found that there is a linear relationship between the normal forces and the threshold displacement amplitude [7]. [PERSON] and [PERSON] developed an empirical equation to account for the impact of fretting frequency or temperature, and proposed failure-time variation sensitivity for each variable [8]. However, very few studied how to quantifiably correlate the tip design with contact electrical performance. This paper proposed a new methodology to correlate contact tip design with contact resistance.