Phil,

Your vendors are correct. With today's liquid photoimagable solder mask, 
which is most commonly applied by screen flood coat, it is difficult to 
insure a deposit beyond 0.0003" on the knee of circuits. Through testing 
over the years, I have found that this coverage is enough to withstand the 
rigors of the rest of the fabrication process as well as assembly. I would 
be careful however, if the coverage drops below 0.0003"  I have found 
difficulties surviving the electroless nickel, emersion gold process as 
well as repeated thermal excursions in assembly.

In the old days of screened defined, thermally cured epoxy and dry film 
solder masks we did get more coverage over the knee due to higher 
viscosity of the liquid masks and the thicker profile of the dry film 
solder masks.

Regards,

Bob Metcalf
Atotech Western Regional Manager

Cell 714-334-7667
E-Mail [log in to unmask] 

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Phil Nutting <[log in to unmask]> 
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09/23/2008 05:28 AM
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[TN] Solder mask thickness






Good afternoon,

OK, here is this afternoon's question.  Solder mask thickness, what is the 
"industry standard"?  We currently specify 0.001" thickness and this may 
be fine on a large area when tested via IP TM-650, 2.5.6.1.  Now use 2 
ounce copper traces in a normal board configuration.  Our board vendors 
are stating that they cannot hold the 0.001" thickness at the edges of the 
etch, it is more like 0.0003".  And if we spec 4 ounce or 6 ounce I'm sure 
it doesn't get any better.

How are you specifying solder mask thickness on your heavy copper weight 
circuit boards?

Do your board vendors use two coats of mask to obtain the thickness needed 
to comply?

IPC-SM-840 section 3.4 says it must meet the requirements of section 
3.8.1, thickness of 0.001" at 500 Vdc.

Your guidance is requested.

Thanks,

Phil Nutting

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