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26 The PCB Design Magazine • April 2015 regard to in/out (I/Os) density. With the value of real estate becoming so important, the space utilized by reflow bonding techniques is becom- ing an obstacle. When discussing real estate, it is common to solve the issue with an expan- sion in the Z-axis. In the property market this is fine, but in the portable electronics market this direction is also becoming crucial. The Z-axis constraints are high-speed signal transfer and signal integrity. The reflow process requires the application of solder. Aside from this procedure being already at the application limit, the utili- zation of solder does not lie within the remit of controlled Z-axis expansion and is also a com- paratively poor conductor compared to copper or other more noble metals. Here we have es- tablished that not only will the I/Os increase in density (decrease in pitch), but the pillar height (standoff height) will also become paramount to future developments. To this end Atotech is cooperating with Georgia Institute of Technol- ogy to assess the viability of using EPAG as the pillar and pad finish for thermo-compression bonding. There exists a prior art to thermo-compres- sion bonding (TCB). These can be characterized in Figure 9. Although the prior art technologies are proven, there are some viable benefits in using EPAG as the copper interconnect. Not only can the key performance indicators or benefits can be compared to direct Cu-Cu bonding, but can be improved upon. Conclusion Although ENIG and ENEPIG still have a place in a supplier's portfolio, the future is look- Figure 7: The performance comparison of the low Ni eNePiG compared to the ePAG process. Figure 8: Performance of ePAG for AG wire bonding. THE FUTURE oF NICkEL IN NICkEL/PALLADIUM/GoLD FINAL FINISHES continues feature