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70 DESIGN007 MAGAZINE I APRIL 2018 give a minor improvement to the drilling pro- cess and electroless copper adhering to the through hole-wall for preparation to copper plating. The glass reinforcement may improve the TCDk and thermal conductivity properties of the laminate. Over time, there have been other improve- ments to PTFE laminates. For instance, adding ceramic filler was a really good idea. However, it can't be just any ceramic filler; if the right ceramic is not selected, some properties will benefit, and other properties will degrade. When the right ceramic filler is used, the CTE is improved dramatically, TCDk improves greatly, and thermal conductivity increases. The improvement to CTE allows these types of laminates to be used in multilayer applications where PTH reliability is important. The pres- ence of the ceramic filler can also improve the copper plating preparation steps. When the right ceramic filler and woven- glass are combined in a PTFE-based laminate, the circuit board fabrication and RF perfor- mance issues can be overcome. The woven glass helps to rigidize the laminate and make it more dimensionally stable. The proper com- bination of ceramic filler and woven glass can improve the CTE, TCDk, and thermal conduc- tivity, and allow simpler drilling and PTH prep- aration processing. Here are a couple of examples. A PTFE-based laminate that has been around for decades, RT/duroid 5880, is made up mostly of PTFE and a small percentage of micro-fiber glass. This laminate has been the gold standard in the industry for many years due to its excel- lent high-frequency performance. However, it does have several inherent drawbacks due to the formulation being dominated by PTFE. When using this material, the applica- tion and interconnect processing capabilities must be considered. The RT/duroid 5880 has excellent high-frequency performance but the Z-axis CTE is about 240 ppm/°C, the TCDk is -125 ppm/°C, thermal conductivity is 0.21 W/mK and the PTH hole preparation must be done with an aggressive wet chemistry. In contrast, the RT/duroid 6202PR laminate is PTFE-based however the formulation uses the right ceramic filler and woven-glass rein - forcement. The Z-axis CTE for this material is 30 ppm/°C, the TCDk is -15 ppm/°C, thermal conductivity is 0.68 W/mK and the through hole preparation can be done with a special plasma process which is typically easier to implement than the wet chemistry used to prepare the PTH. Just to provide some reference to these num- bers, a good rule of thumb for CTE is to be at 70 ppm/°C or less, while keeping the TCDk at 50 ppm/°C or less, and thermal conductivity is considered good when it is higher than 0.50 W/mK. Avoiding the appropriate wet chem- istry for PTH preparation can benefit circuit manufacturing yields. The circuit board designer must be aware of the different properties of these materials to ensure they are used in applications and PCB fabrication processes which are appropriate for the material. Communicating with your laminate supplier and PCB fabricator early in the design process is more critical than ever when working with PTFE and PTFE-based materials. DESIGN007 John Coonrod is technical marketing manager for Rogers Corporation. To contact him or view past columns, click here. When the right ceramic filler and woven-glass are combined in a PTFE-based laminate, the circuit board fabrication and RF performance issues can be overcome.

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