Good thermal pads can be used. (But not the ones delivered with the cooler. They are quite poor).
Thermal grease is better because the layer should always be thinner than a pad.
I recommend low viscosity thermal grease (if you are from Germany you can use item no. "LEITPASTE 20GR" from
http://www.reichelt.de) and the "blob in the center" method. It is simple, quick and save. I use this method for testing new batches and never had any problems.
I've tried Arctic MX 2. It's better than Artic Silver 5, but I could not experience such a huge step like Chefnet:
or better cooling, my single went up from 204 to 220 with artic mx-2 under the cooler
... usually I get 4-8 Mhz more.
002-0: ztex_ufm1_15y1-04A347274B-1: f=200.00MHz, errorRate=0.00%, maxErrorRate=0.58%, hashRate=200.0MH/s, submitted 12 new nonces, luckFactor=0.99
002-0: ztex_ufm1_15y1-04A347274B-2: f=204.00MHz, errorRate=0.00%, maxErrorRate=1.52%, hashRate=204.0MH/s, submitted 17 new nonces, luckFactor=0.97
002-0: ztex_ufm1_15y1-04A347274B-3: f=196.00MHz, errorRate=0.00%, maxErrorRate=0.81%, hashRate=196.0MH/s, submitted 10 new nonces, luckFactor=1.03
002-0: ztex_ufm1_15y1-04A347274B-4: f=200.00MHz, errorRate=0.00%, maxErrorRate=1.18%, hashRate=200.0MH/s, submitted 19 new nonces, luckFactor=1.02
---
002-0: ztex_ufm1_15y1-04A347274B-2: f=208.00MHz, errorRate=0.00%, maxErrorRate=0.50%, hashRate=208.0MH/s, submitted 9 new nonces, luckFactor=0.96
002-0: ztex_ufm1_15y1-04A347274B-3: f=200.00MHz, errorRate=0.00%, maxErrorRate=0.62%, hashRate=200.0MH/s, submitted 11 new nonces, luckFactor=0.98
002-0: ztex_ufm1_15y1-04A347274B-4: f=204.00MHz, errorRate=0.00%, maxErrorRate=0.99%, hashRate=204.0MH/s, submitted 15 new nonces, luckFactor=0.96
002-0: ztex_ufm1_15y1-04A3476278-1: f=208.00MHz, errorRate=0.65%, maxErrorRate=3.22%, hashRate=206.6MH/s, submitted 18 new nonces, luckFactor=0.96
@Chefnet: How do you apply the grease? A blob in the middle? How much? Moving the heatsink around, or just pushing it down?
The one problematic board still sucks. Hashrate went up:
002-0: ztex_ufm1_15y1-04A3476278-1: f=200.00MHz, errorRate=0.87%, maxErrorRate=1.00%, hashRate=198.3MH/s, submitted 14 new nonces, luckFactor=0.82
002-0: ztex_ufm1_15y1-04A3476278-2: f=196.00MHz, errorRate=0.23%, maxErrorRate=0.55%, hashRate=195.5MH/s, submitted 16 new nonces, luckFactor=0.88
002-0: ztex_ufm1_15y1-04A3476278-3: f=196.00MHz, errorRate=0.00%, maxErrorRate=0.81%, hashRate=196.0MH/s, submitted 13 new nonces, luckFactor=0.83
002-0: ztex_ufm1_15y1-04A3476278-4: f=192.00MHz, errorRate=0.11%, maxErrorRate=0.50%, hashRate=191.8MH/s, submitted 10 new nonces, luckFactor=0.95
---
002-0: ztex_ufm1_15y1-04A3476278-1: f=208.00MHz, errorRate=0.81%, maxErrorRate=3.22%, hashRate=206.3MH/s, submitted 16 new nonces, luckFactor=0.96
002-0: ztex_ufm1_15y1-04A3476278-2: f=204.00MHz, errorRate=0.26%, maxErrorRate=1.20%, hashRate=203.5MH/s, submitted 17 new nonces, luckFactor=1.06
002-1: ztex_ufm1_15y1-04A3476278-3: f=204.00MHz, errorRate=0.04%, maxErrorRate=3.01%, hashRate=203.9MH/s, submitted 15 new nonces, luckFactor=0.98
002-1: ztex_ufm1_15y1-04A3476278-4: f=196.00MHz, errorRate=0.00%, maxErrorRate=0.56%, hashRate=196.0MH/s, submitted 10 new nonces, luckFactor=1.06
...but No.4 still is below 200 Mhz and I'm getting FPGA-shutdowns (5 times since yesterday):
002-0: ztex_ufm1_15y1-04A3476278-1: Error: Hash rate drop of 5.8% detect. This may be caused by overheating. FPGA is shut down to prevent damage. 51.99384114688102: Device disabled since 2012-05-02T03:05:59
002-0: ztex_ufm1_15y1-04A3476278-2: Error: Hash rate drop of 5.4% detect. This may be caused by overheating. FPGA is shut down to prevent damage. 51.772818289686384: Device disabled since 2012-05-02T02:57:22
. Speed is as expected. Thanks god no 196 chip... 1x 212, 2x 208 and 1x 204 MHz with the stock fans. I plan to replace the 40mm fans with two 120mm fans, one from above and one from below plus some RAM coolers on the bottom of the chips. Would be nice if I could gain some extra speed. 
