Friday, March 09, 2007

Highspeed design

At first glance, it would seem that the Proadlizer's or X2Y caps are worth it. More discussion is on c.a.f, which also points to the SI-list. IDC caps are another choice.

In a semi-related discussion: some have claimed that ISOLA FR408 (Dk of 3.7) rates very close to N4000-13 SI (Dk of 3.5 to 3.7, 0.009 loss @ 10G), but most discussions I've seen seem to indicate it is a noticable step below. It definitely costs less though, so it might be a decent tradeoff. Not that 13SI is ideal: it may have delam problems at RoHS soldering temperatures. PLC621, Matsushita Megtron 5 and IS620 (Dk of 3.5, Df of 0.008 @ 10G) are matches to SI and are also supposedly RoHS compatible. IS415 may be a decent 10G match as well. Also, 4000-12 might be a valid alternative to 4000-13 for Pb-free processes, if I could find the specs for it. Oh, and don't forget about moisture absorption. Lastly, avoid the use of Pure tin plated compoents if possible, otherwise try to source parts with post Sn plate annealing of the suface finish. Other good articles.

IEEE802.3ap Improved FR-4 (Mid Resolution Signal Integrity):
–100Mhz: Dk≤3.60;Df≤.0092
–1Ghz: Dk≤3.60;Df≤.0092
–2Ghz: Dk≤3.50;Df≤.0115
–5Ghz: Dk≤3.50;Df≤.0115
–10Ghz: Dk≤3.50;Df≤.0125
–20Ghz: Dk≤3.40;Df≤.0140

Temperature and Humidity Tolerance (0-55degC, 10-90% non-condensing):
–Df: +/-.001

Resin Tolerance (standard +/-2%):
–Df: +/-.0005

These slides seem to show certain no-Fr4 materials being close to the same price, but considerable improved 10G performance: Nelco 6000, TacPreg, Taconic RF35, and Rogers 4350. I'm sure there are some trade-offs though... yep, the DDI chart shows them to not be quite so good in thermal performance.

  • Td - Thermal decomposition temperature
  • Tg - Glass transition temperature
  • CTE - Coefficient of thermal expansion
  • IST - Internal Stress Test


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