Page 54


www.us- tech.com


December, 2019


Increasing Signal Integrity in HDI, 5G, High-Frequency PCBs


By Kunal Shah, Ph.D., President, LiloTree


internet, and handheld/wearable devices, the amount of information transfer across wireless networks has increased enormously. To trans- fer the amount of data required by today’s technology, electronic device manufacturers resort to high-speed, high-frequency electronic signals. The integrity of high-frequency


W


signals can be affected by the choice of materials used to fabricate PCB assemblies in receiving devices. The combination of high-frequency sig- nals with small conductive traces, such as wearable devices, leads to signal losses and compromised per- formance.


Signal Loss The primary factor affecting the


integrity of high-frequency signals is conductor loss, mainly dependent on the surface finish applied on the cop- per pads of PCBs. Among the surface finishes available for these applica- tions, electroless nickel immersion gold (ENIG) has been a popular choice. However, ENIG can lead to an


increase in insertion loss, due to the presence of nickel. Nickel has 1/3 the conductance of copper, leading to high- er conductor losses. Also, the nickel-


ith the tremendous growth of mobile phones and de - vices, the availability of the


phosphorous (Ni-P) layer has ferro- magnetic properties that adversely affect the circuit performance. Figure 1 shows increased inser-


tion loss, due to ENIG surface finish on copper conductors compared with bare copper over a 0 to 50 GHz fre- quency range. The latest generation


using 8 mil thick 0


0.5 1


Bare CU 1.5 CBCPW bare CU


2.5 2


3 0 5


Tightly coupled 10 15 20 25


of cellular network, 5G, uses millime- ter frequency bands at the higher end of that range. For example, Verizon is using a 29 GHz band, while AT&T is using a 39 GHz band. Higher-GHz bands are already being discussed for cell networks between 50 to 100 GHz, due to the higher ,


, p laminate


throughput of data available at high- er frequencies. Automotive radar already uses


a 76 to 81 GHz band. This creates an issue that must be addressed not only for fast-approaching future tech- nologies, but also for current devices that still have to contend with signal losses, due to the nickel in ENIG sur- face finishes. Already, the market is using a number of high-frequency circuit boards, which is one of the fastest-growing areas in the electron- ics industry.


Nickel-Less ENIG-Premium With the growing need for circuit


ENIG CBCPW ENIG 30 35 40 45 50 Frequency (GHz) Figure 1: insertion loss comparison between GCPW circuits with


bare copper conductors and with ENIG-plated copper conductors, from 0 to 50 GHz. Line width: 11 mil, space 6.5 mil, 8.5 mil thick laminate.


boards and designs for high-frequency applications, the insertion loss due to ENIG surface finish is becoming unacceptable in the industry. For these types of applications, a new sur- face finish must be developed. First, a replacement for ENIG


in this space must contain no nickel. This is to eliminate the high inser- tion loss and ferromagnetic element of ENIG. Second, it should have a final gold finish to retain the high reliability and long shelf life rating that users expect from ENIG. Third, it should exhibit greater reliability (robust solder joints) and be cost- effective (minimal use of precious


Continued on page 57


Loss (dB/in.)


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