FreeLists / si-list / [SI-LIST] edge coupled coated microstrip differential impedance

[jerry_hu@xxxxxxxxxxxx]

Hi, Gurus,
I have couple of questions regarding the edge coupled coated microstrip 
differential impedance. 

Recently, I have designed a board with 100 Ohm differential impedance 
microstrip. The design track width is 7mil, 4mil space edge to edge, 8.9 
mil distance to the reference plane. The track thickness is 0.5oz plus 1oz 
plating. one local board shop made the boards for us. The actual track 
width is 7mil, 4mil space and 8..4 distance to the reference plane. The 
track thickness is 0.5oz plus 1 oz plating. The measurement for these 
differential pairs on a coupon (6 inches) is 101 Ohm, which is good. 

We switched another board shop for the second spin. they told us that the 
spacing have to be adjust from 4mil to 7mil in order to make 100 ohm 
microstrip differential pair. Their calculations show the impedance with 
my original design would give around 80 ohm. the parameters they were 
using are, track width 7mil, spacing 4mil, 9.5mil distance to the 
reference plane, track thickness 0.5mi plus 1.5 plating, 0.8mil solder 
mask. The dielectric constant is 4.7 for their calculation.

Here are my questions:

1) why is the calculation and result so different between these two board 
shop?

2) how much will solder mask effect the differential impedance on 
microstrips? I know solder mask has some impact to the microstrip 
impedance. Is that possible to be 20% difference?

3) If the solder mask or material or processing have such big impact to 
microstrips, why the first board shop doesn't need to adjust to get 100 
ohm? 

My calculation is based on the edge coupled microstrip without counting 
the solder mask coating. I thought it would be close enough for board shop 
to adjust tracks according to their processing and make make boards. But 
this makes me to rethink about this idea. Anyone has such experience?

thanks and regards.

Jerry Hu



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