A Study of Impedance Control in Flexible Printed Circuit (FPC)

Transcription

1 A Sudy of Impedance Conrol in Flexible Prined Circui (FPC) Tomonari Yokoyama 1, Hiroio Waanabe 1, Yukiaru Sakaguci 1, Haruo Miyazawa 1, Yosiaru Unami 1 and Nobuyuki Sadakaa 1 Ordinary, requiremens for prined circuis are only o be conneced correcly and o ave enoug elecrical curren capaciy. If signals used inside of equipmens are very low frequency, impedance conrol of circui is no needed. Impedance conrol is needed only for long communicaion cables beween equipmens. Wen elecrical circuis wi ig frequency signal are operaed, impedance conrol is needed for no only communicaion cables bu also prined circuis. Recenly i is no rare a e ig frequency signal (several undreds MHz o several GHz) is used inside of equipmen. In is case, e circui sould be considered as disribued consan circui even if e circui is very sor. Terefore impedance conrol of circui is needed on Prined Circui Board. Based on e background described above, we ave ried o conrol caracerisic impedance of Flexible Prined Circui (FPC). 1. Inroducion In e field of elecronic circuiry, if necessary wires were properly conneced on a prined circui board and e wiring paern ad a sufficien curren capaciy relaive o e curren, no rouble occurred in e operaion of e prined circui board. Along wi e recen remarkable increase in e circui operaion speed, owever, e above-menioned condiions are no sufficien for e design of a prined circui board. One of e required iems is e caracerisic impedance of a paern. So far, for a device using only a low-speed signal of several MHz or less, only e caracerisic impedance of elecommunicaion cables connecing beween e devices sould be considered. Tis was because e waveleng of e signal o be used was long enoug compared o e size of e device, and e circui in e device could be regarded as a lumped consan circui. Te cables connecing e devices are no sufficienly sor in relaion o e waveleng a is o be used, ence i sould be regarded as a disribued consan circui. Terefore, conrol of e caracerisic impedance is required for cables. Recenly, i is no uncommon o use elecrical signals wi e frequency of several 1 MHz o several GHz in a device. If suc a ig-speed signal is ransmied, a ransmission line sould be regarded as a disribued consan circui even if e ransmission 1 Circui Tecnology Developmen Division, Elecronic and Elecrical Developmen Cener line is sor and inended for use only in e device. Terefore, e conrol of e caracerisic impedance on e prined circui board is required. In lig of e above siuaion, a flexible prined circui (ereinafer referred o as FPC) is growing in demand. Tis repor describes e callenges in conrolling e caracerisic impedance on FPC and e presen acievemens.. Ouline.1. Caracerisic impedance Caracerisic impedance is deermined by e square roo of e raio beween e inducion elemen and e capaciy elemen of e signal line per uni leng. If e oupu impedance of e signal oupu circui, e load impedance conneced o e erminal porion of e signal line, and e caracerisic impedance of e ransmission line are consisen, no signal reflecion occurs on e ransmission line. Te connecion of inpu/oupu impedance a is equal o e caracerisic impedance is equivalen o an infiniely coninual ransmission line aving a uniform caracerisic impedance... Hig-speed signal ransmission line Te signals ofen used for a ig-speed daa ransmission are PECL, LVDS, CML, ec. For ese signals, a driver for connecing e load of 5 Ω of impedance is used and e oupu impedance is adjused o 5 Ω. On suc an inerface, e caracer- Fujikura Tecnical Review, 6 3

2 isic impedance of e ransmission line o be conneced needs o be conrolled o 5 Ω.3. Differenial impedance In suc a ig-speed signal, differenial inpu and oupu are generally used o enance resisance o e common mode noise from e ouside. Inpu impedance a e erminaion of e ransmission line is also defined by differenial impedance, wic is 1 Ω. In is case, e caracerisic impedance of e ransmission line is also defined by e differenial impedance, wic is 1 Ω, ereby prevening signal reflecion beween ransmission lines. Generally, differenial impedance is no equal o e sum of e caracerisic impedances of wo signal wirings. Te larger e coupling coefficien beween wo signal wires, e smaller e differenial impedance. If ere is a large gap beween wo signal wires so a i may be regarded as no connecion of wo wires, e differenial impedance becomes equal o e sum of e caracerisic impedances of wo signal wires. 3. Simulaion 3.1. Modeling of ransmission line To conrol e caracerisic impedance on e wiring board, a co-planar line, microsrip line, or srip line is generally used Co-planar line A co-planar line as a srucure of e ype of ransmission line a is used for e cables suc as a feeder line. I is formed by a parallel wiring so a e gap beween e circui paern ransmiing a signal and e circui paern of e ground becomes consan. Te weak poins in using a co-planar line are a e degree of freedom for wiring becomes low because e impedance is conrolled only by e wid and e gap, and e noise radiaed from e signal line becomes greaer an in a microsrip line or a srip line because of e line wiring a e ground Microsrip wiring On a microsrip line, signal paerns are arranged o overlap a ground plane. A co-planar line can be made up of a single-sided board, wereas a microsrip line requires a board aving a leas wolayers. On a microsrip line, e caracerisic impedance can be conrolled by e disance beween e ground plane and e signal paern and e wiring wid of e signal paern. Tis is a ransmission line a is relaively easy o connec, can reduce radian noise, and can be used mos frequenly on e prined circui board Srip line A srip line as a srucure were signal paerns are arranged so as o ge caug in e ground plane. Terefore, e board mus ave a leas ree layers. Similar o a microsrip line, e caracerisic impedance can be conrolled by e disance beween a ground conducor and a signal paern and e wiring wid of e signal paern. Tis is a srucure a can reduce e larges amoun of radian noise. ε re 1 + (a) Co-planar line 3 π K 1 { w/ ε K{ w/ re 1 K { w/ K 1 { w/ (b) Microsrip line ln. 475 ε w + K r (c) Srip line ln ε π ( 8. w + ) r s K w sin ( πw/ 4) sin { π / 4 ( ) sin πw/ 4 1 sin { π / 4 w W w : Signal paern wid s : Disance beween ground plane and signal paern : Tickness of dielecric maerial layer : Tickness of conducor layer : Permiiviy Fig. 1. Srucure of ransmission lines. s 3) ) 1) 4

3 Differenial line Te differenial impedance is conrolled by e gap beween signal paerns, oo. W D1 G 3.. Convenional analysis meod On ese ransmission line models, e caracerisic impedance of co-planar ransmission line, microsrip ransmission line, and srip ransmission line configured on e FR-4 (glass, epoxy) board as been analyzed. Bu e caracerisic impedance wi a rigorous elecromagneic field analysis required a very complicaed calculaion, and e calculaion meod using approximae expressions as indicaed in Fig.1 was generally used o calculae e caracerisic impedance. Tese maemaical expressions, owever, are approximae expressions derived from e acual measured values of e caracerisic impedance. Terefore, e accuracy of e approximae values on inducion raio, board ickness, and signal paern wid a are no usually used on e FR-4 board is low, and ese approximae expressions canno be used for e calculaion of e caracerisic impedance of FPC. (a) Co-planar line W (b) Microsrip line W G Inroducion of simulaor Along wi remarkable improvemen in e operaion speed of e elecronic circui, conrol of caracerisic impedance was required. A e same ime, along wi remarkable improvemen in e processing ime of e compuer, a rigorous elecromagneic field analysis of ransmission lines roug numerical calculaions became possible. Under e circumsances, e elecromagneic field analysis simulaor of ransmission lines was inroduced. Tis allowed conducing numerical calculaions accuraely even on e parameers were e accuracy of e approximae values becomes low if e approximae expressions for e FR-4 board as Fig. are used Impedance simulaor on FPC Regarding a flexible prined circui board, i is necessary o consider e poin a i is no made up of a single insulaion maerial suc as FR-4, bu of composie maerials suc as an adesive layer and a base maerial layer. Te difference of e conribuion raio o e impedance values of eac composiion of microsrip line suc as layer ickness and wid is measured roug e simulaion. I was found a insulaion layer ickness and signal paern wid ad e larges effec on e impedance. 4. Feedback by e acual measuremen H Er (c) Srip line,w : Signal paern wid G1,G : Ground paern wid D1 : Disance beween ground paern and signal paern,h : Tickness of dielecric maerial layer : Tickness of conducor layer,er : Permiiviy 4.1. Acual measuremen of caracerisic impedance by TDR To correlae e calculaion resul roug e simulaion using e microsrip line model wi e caracerisic impedance of e acual FPC, we made e samples of componens suc as copper clad laminaes (ereinafer referred o as CCL) and cover lay (ereinafer referred o as CL) using various maerials and assigned a signal paern wid and measured e caracerisic impedance using TDR. 4) 4.. Cross-secional observaion of samples To give a deailed feedback according o e simulaion resul, a cross-secion observaion was conduc- Fig.. Srucure of ransmission line by simulaor. Fujikura Tecnical Review, 6 5

4 CL Base maerial ickness CL Adesive H5 ickness Paern op wid H4 W CCL Copper ickness T CCL Paern boom Adesive ickness wid H3 CCL Base maerial ickness H CCLAdesive ickness cracerisic impedance (Ω) signal paern wid (µm) FPC1 FPC FPC3 FPC4 FPC Fig. 4. Calculaed caracerisic impedance. Fig. 3. Cross secion view of FPC. ed on e FPC were e caracerisic impedance was measured. Te measured poins on e crosssecion observaion were: (1) CCL base maerial ickness, () CCL adesive ickness, (3) signal paern op wid, (4) signal paern boom wid, (5) signal paern copper foil ickness, (6) CL base maerial ickness, and (7) CL adesive ickness. Figure 3 sows e cross-secion poo of e FPC Comparison of e resuls By comparing e simulaion resul and e acual measuremen of e caracerisic impedance based on e acual measured parameers and e acual measured permiiviy of e base maerial, e correlaion wi e acual circui was examined o confirm e effeciveness of e simulaion. Te resul sowed a e difference, depending on e kinds of maerials (adesive agens and base maerials are differen depending on e maker), was small, and a i depended eavily on e srucure suc as e layer ickness and signal paern wid. Tese resuls corresponded o e esimaion roug e simulaion. Wi regard o e impedance values oo, ere was a cerain raio of agreemen beween e simulaion values and e acual measured values. From ese facs, i was found a wi compensaion values, e simulaion could be applied o e FPC similarly on e FR-4 board. Figure 4 sows e caracerisic impedance a was calculaed using e simulaion sofware, and Figure 5 sows e caracerisic impedance a was acually measured. 5. Impedance conrol in a manufacuring process Based on e above-menioned resuls, we examined e flucuaions of various facors affecing e impedance value in e acual mass-producion manufacuring process and evaluaed weer impedance cracerisic impedance (Ω) could be conrolled. Especially we cecked e insulaion layer ickness and e signal paern wid a were expeced o affec e caracerisic mos significanly. Concerning e maerial ickness, e daa submied roug e maerial supplier sowed a e flucuaions affeced e impedance only a a slig level. Ten i was found a e effecs of e signal paern wid on impedance depended on weer e flucuaions in e ecing process could be reduced, and in addiion, e ickness of e plaing and paern layou affeced i. We made i possible a e caracerisic impedance was conrolled wiin ±1% of e arge value and e differenial impedance was conrolled wiin ±15% in consideraion of e flucuaions in maerials and manufacuring processes. 6. Conclusion signal paern wid (µm) FPC1 FPC FPC3 FPC4 FPC Fig. 5. Measured caracerisic impedance. To conrol e caracerisic impedance of e FPC, we conduced simulaions on e ransmission line models and made rial models o measure e acual values. Te microsrip line srucure could realize e arge caracerisic impedance, and e design values of e paern wid could be calculaed. On e FPC, e conrol of e signal paern 6

5 wid ad e larges effec in various parameers, and i was found a i was imporan o reduce e flucuaions in wid in e layou of e circui. We made i possible o conrol e impedance including e flucuaions in maerials and signal paern wid in mass-producion. Wi e promoion of ig-densiy packaging and wiring using e miniaurized and iger-performance porable devices, in, flexible, and iger-definiion wires are required for a flexible circui board. I is difficul o mee e impedance specificaion wi a in maerial on e convenional srucure. So, we are now sudying on some meods including e formaion of e ground conducor layer of a microsrip line ino a mes. Finally, o womsoever i may concern, we ank you for your guidance and cooperaion in associaion wi is sudy. References 1) Reuli: Inducance Calculaions in a Complex Inegraed Circui Environmen, IBM J. Res. Develop., 16, 8, 197 ) Cerensky, Senire, Modi: Ceramic muli-layered circui board wi greaer conrollabiliy of elecric properies and reduced noise, Te Nikkei Elecronics ) Noboru Sibuya, Masasige Masumoo, Tosiki Jakube: Inroducion o muli-layered prined circui board ecnology 1, Te Circui Tecnology Vol., No., ) TDRTime Domain Reflecmeory Fujikura Tecnical Review, 6 7