"The Effect of FR-4 Laminate Materials on the Surface Insulation Resistance of Wave Soldering Fluxes" Authors: Karen Tellefsen, Anna Lifton, Mike Previti and Mike Murphy; firstname.lastname@example.org.
The most likely reason for these results is differences in FR-4 laminate chemistry used in the test coupons because of the higher processing temperatures used in Pb-free assembly.
Actually, we should be asking when FR-4 will reach its limits, and what will we do about it?
The limitations of FR-4 are most noticeable in high-speed serial links with data rates in excess of 2.5 Gbps and in backplane traces longer than 12 inches.
Rise time degradation sets the ultimate limit to the data rate that can be transmitted in FR-4. It gives rise to effects such as intersymbol interference (ISI), collapse of the eye diagram and deterministic jitter.
XAUI was defined to extend the optical module system interface reach inside a network element up to 20 inches through a standard FR-4 board and two connectors.
In the test with Tyco Electronics performed at the UNH InterOperability Laboratory, the Mindspeed SkyRail(TM) SerDes transceiver successfully passed and recovered data at full speed through 27 inches of standard FR-4 backplane material and two Tyco Z-PACK HM-Zd connectors with no errors for a minimum of five minutes with other SerDes transceivers.