[SI-LIST] Resistivity of Tin/lead Plating

I had to design a bacplane board having many thick power tracks. I
calculated the width of the tracks (for a required voltage drop) with
respect to 1 oz (or 35 micron) copper weight. But the PCB fab told me that
they woould deposit anadditional 10 micron of copper on the tracks during
electroless copper deposition process. After that another 25 micron of
Tin/Lead  would be deposited on the copper during tin plating.
So the ultimate thickness of copper would be more than I specified. Can
someone tell me what value should be substituted for the resisitivity of
Tin/Lead to calculate the resistance of the whole track ?.

My second question is about the Solder Masks.
There are usually 3 types of Solder Mask available from the PCB
manufacturers, namely, LPI, Dry Film and Wet. LPI is mostly suited to track
widths in the range of 5-8mils but is less thicker then dry film. Dry film
is more thick but at the same time more expensive. Can someone comment on
the reliability issues of these different Solder Mask types and in which
application one should be preferred on another ?.

Lastly, I have a question regarding various surface finishes using Gold.
Usually the manufacturers support following types of surface-finish with
Gold.
1. Panel Gold Plating (Flash Gold) 
2. Electroless Nickel Immersion Gold
3. Normal Gold Finger Plating (usually carried out for PCI cards)

Can someone comment on the advantages and disadvantages of these different
types of plating ?.



ADEEL MALIK,
Design Engineer,
Communications Enabling Technologies,
Software Technology Park,
5-A Constitution Avenue,
Islamabad, Pakistan.
FAX: 92-51-2827469
TEL:  92-51-2826160

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