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62 The PCB Design Magazine • April 2014 A combination of FR-4 materials and high- frequency hydrocarbon laminates typically has few circuit fabrication issues. The main concern with this combination of materials is typically drilling and lamination. A DOE (design of experiments) is usually necessary to establish the proper feeds/ speeds for drilling a circuit of this combination of materials. Lamination can be an issue for FR-4 prepreg, which often requires a ramp rate that is very different than the high- frequency prepreg. In order to make a more reliable hy- brid, there are a couple of op- tions to consider when using FR-4 and hydrocarbon prepreg. One option is to replace the FR-4 prepreg with the high-frequency prepreg and use the appropriate lamination cycle. A high- frequency prepreg is typically not as expensive as the laminate, and having all bonding layers made of the same material is beneficial for a simpler lamination cycle. When the FR-4 pre- preg cannot be replaced, it may be necessary to do sequential lamination, where the first lami- nation cycle cures the FR-4 prepreg and the sec- ond lamination cycle cures the high-frequency prepreg. A hybrid multilayer PCB using a combina- tion of FR-4 and high-frequency PTFE circuit materials can be more challenging, however there are some exceptions. There are several dif- ferent types of PTFE-based laminates and some are easier to fabricate than others. Even though ceramic-filled PTFE-based laminates have less circuit fabrication concerns than a pure PTFE laminate, drilling, PTH preparation and dimen- sional stability must be considered. The main concern about PTH drilling is that the PTFE material is relatively soft and the FR-4 material is rigid. When the PTH is drilled and the drill tool goes through the interface of these material boundaries, the hole can have a flap of soft material stretched over some length with- in the PTH wall. This can be a serious reliabil- ity issue and must be addressed. Typically the proper feeds and speeds must be determined through a DOE and a study regarding drill life. Many times the flap defect does not occur early in the drill tool life, so understanding the drill life is important for minimizing this concern. The PTH preparation must address both types of materials for the plated through-hole process. The plasma cycle will prob- ably need to be two different cycles or one cycle with mul- tiple stages. The FR-4 materials should be treated in the first plasma cycle and the PTFE materials in the second cycle. Typically, the plasma process for FR-4 uses CF 4 -N 2 -O 2 gases, and PTFE uses He or N 2 H 2 . For improved through-hole wall wettability, the recommended cycle using helium (He) is a bet- ter choice for treating PTFE. If wet processing is used to prepare the PTH, perform the permanganate first for the FR-4 materials followed by sodium naphthalene to treat the PTFE material. Dimensional stability or scaling can be an issue with PTFE and FR-4 hybrids. This can be reduced by minimizing mechanical stress of the PTFE laminate. Scrubbing the panel induces random mechanical stresses and is not recom- mended. A chemical clean process is a better method for preparing the copper for subse- quent processing. The thicker PTFE laminates will have less issue with dimensional stability and the woven-glass reinforced PTFE substrates will be more stable as well. In general, the manufacture of hybrid PCBs with a combination of FR-4 and high-frequency circuit materials has few compatibility issues. But several circuit fabrication concerns need to be addressed. When working on hybrid multi- layer builds, it is always recommended that the fabricator consult with the material manufac- turer for best results. PCBDESIGN lightning speed laminates A high-frequency prepreg is typically not as expensive as the laminate, and having all bonding layers made of the same material is beneficial for a simpler lamination cycle. " " John Coonrod is a market de- velopment engineer for rogers Corporation, Advanced Circuit Materials Division. To reach Coonrod, click here. HyBRID HIGH-FREQuENCy MuLTILAyER PCBS continues