A Low Loss PTFE-based Bond Ply Material for Multilayer ... - Taconic

PRODUCT FEATUREA LOW LOSSPTFE-BASED BONDPLY MATERIALFOR MULTI-LAYERPCB APPLICATIONSGrowing needs are developing for purepackage high performance, low losslaminate and prepreg materials for microwavetelecommunications and high speeddigital printed circuit board (PCB) applications.Polytetrafluoroethylene (PTFE), perhapsbetter known by the DuPont trade name,Teflon, has an over 30-year long history ofmeeting the needs in microwave frequencyapplications. PTFE-based materials reinforcedwith woven glass fabric and ceramicfillers offer the advantages of very low loss

This prepreg, when combined withthe corresponding low loss laminate,provides the design engineer with apure package approach for realizingexceptional electrical performance.The overall composite contains from10 to 20 percent of a flowing thermosettingresin. The result is a compositethat is sufficient to deliver a lossat 10 GHz of less than 0.005, peelstrengths from 5 to 6 lbs, (sufficientlygood to do foil laminations), good dimensionalstability that enable theproduction of 20 to 40 layer countmulti-layer circuits, gap filling of 1.0oz. circuitry and drill performance exceedingstandard PTFE boards. Becausethe composite is a hybrid ofPTFE and a thermosetting resin, thecomposite has a rigidity similar toepoxy. The woven fiberglass reinforcedcomposite enables the productionof very high layer count PCBswith good layer-to-layer registrationPRODUCT FEATURETABLE ITP-32 PRELIMINARY DATAProperty Typical Value MethodGlass transition (°C) 172.73 DSCPressed thickness (mil) 3.2 to 3.7**Dielectric constant (tba)at 10 GHz3.29 IPC-TM 650 2.5.5.5Dielectric loss at 10 GHz 0.004 to 0.005 IPC-TM 650 2.5.5.5Moisture absorption (%) 0.014 to 0.240 IPC-TM 650 2.6.2.1Peel strength (1.0 oz. ed)(lbs/linear inch)because the processingat epoxyprocessing temperaturescauses verylittle movement inthe predominantlyPTFE-based composite.TacPreg is apatent pendingtechnology that definesthe compositemixing of thermosetresin withPTFE/wovenfiberglass. The firstTacPreg product isTP-32 with a dielectricconstant of 3.20 and loss tangent0.005. Table 1 outlines the TP-32 materials’ preliminary specifications.A 20-layer 0.190" thick 18" × 24"test vehicle was chosen to evaluatethe material’s process capability, electricalperformance and thermal reliability.Surface foil lamination, variousline widths down to 0.006" with 1 oz.copper and dielectric spacing of0.008" using RF-35-008-C1/C1 laminateand TP-32-003 TacPreg bondingply were chosen for the constructionmaterials. The RF-35P laminate materialsare available down to 0.002"thickness.A series of prototype PCBs weresuccessfully built and the informationgathered from the boards continuesto be evaluated. Additional tests continueto progress toward the developmentof more detailed processingwindow parameters. What follows is abasic overview of the results of theseefforts. Specific detailed recommendationsare available through thecompany for qualified project work.5 to 6 IPC-TM 650 2.4.8**Pressed thickness will vary depending on copper trace density**and type of copper (0.5 to 2.0 oz.).▲ Fig. 1 A magnified cross-sectionshowing the TacPreg materialfilling the internal circuitry.MATERIAL EVALUATIONThe RF-35P laminate cores wereprocessed using standard print andetch techniques. Standard copper adhesionpromotion treatment was appliedfor bond enhancement. Thelayers were tested through standardautomated optical inspection equipment.For this test vehicle, therewere no compensation factors used toevaluate the dimensional stability ofthe cores. It is anticipated that somecompensation factors will need to bedeveloped at the fabricator level forthe different core thickness. Figure 1shows a magnification of the TacPregfilling of internal circuitry.The boards were constructed withfour plies of the TP-32 TacPreg betweeneach core and to the surfacewith copper foil. The boards werelaminated with an 8°F/min heat riseto the hold temperature of 400°F usingpressure of 400 psi. The holdtime was two hours and the cooldown rate 10°F/min. The boardsshowed good filling and adhesion.The surface copper foil had peelstrength of 6 lbs/in.The boards were drilled usingcommon practices; standard industryavailable drills, entry materials, andfeeds and speeds for thick multi-layerboards. Specific drills, entry andbackup, and feed and speed recommendationsare being developed forfabrication guidance. The boardswere plasma etched to treat the holewall for plating adhesion promotion.A plasma cycle commonly used forPTFE materials in combination withthermoset resin constructions yieldedgood copper plating coverage.The use of RF-35P and TacPregTP-32 in this complex 20 layer count,0.190" thick signal integrity printedcircuit board has demonstrated a viablenew low loss material for achievinghigh performance microwave anddigital integrated functionality. Thelow temperature standard processingconditions for these materials willmake them widely available to theOEM through the general printed circuitboard fabrication infrastructure.TacPreg TP-32 is currently being producedon a preproduction basis and isavailable in limited quantities. VisitTaconic at MTT-S booth no. 423.Taconic AdvancedDielectric DivisionPetersburgh, NY, (518) 658-3202or (800) 833-1805www.taconic-add.com