[SI-LIST] Re: surface finish
- From: "Lee " <leeritchey@xxxxxxxxxxxxx>
- To: <kloebbicke@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 11 Oct 2013 09:00:24 -0700
A simple simulation will show that solder mask has little effect on loss or
impedance of surface traces. As has been pointed out by others, the only
surface finish that has a significant effect on performance is one that
contains nickel and, then, only on very high data rate signals.
There are more important things to worry about that this, such as etching
and plating accuracy.
-----Original Message-----
From: Kai Löbbicke
Sent: Friday, October 11, 2013 8:30 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: surface finish
Hi Randy,
In my opinion, soldermask is also a kind of surface finish. It prevents
the copper traces from oxidation and other influences. For
high-frequency applications, most engineers are afraid of high
dielectric losses due to soldermask. Additionally you need to consider
the soldermask dielectric in the impedance calculation for the trace
dimensions (depends on the frequency range and field distribution). So
many engineers use other surface finishes like Au, Ag, OSP or ASIG. Like
already said in other mails of the SI-list, try to avoid using NiP-Au
surface finishes, because of the worse conductivity of Ni.
To add any surface finish to the copper traces, the outer copper
surfaces were roughened to get a good adhesion. The MEC etch process and
the brushing process are the most common ones. Even if you keep the
outer surface very smooth, the losses do not increase, because the field
distribution is concentrated on the bottom of the microstrip. I verified
that by measurement up to 100GHz. The most loss-generating surface
roughness on the bottom of the microstrip is not influenced.
I measured microstrip lines up to 100GHz with soldermask and the other
surface finishes. Except for the NiP-Au finish, there is almost no
difference in the transmission loss up to 25GHz. The loss depends even
more on the dielectric losses of the substrate. Maybe there exist
different soldermask chemicals, so it could be good to ask your PCB
supplier for a low loss soldermask. So I don`t see any reasons not to
choose soldermask. For higher frequencies, I reached very good results
with a silver surface finish, always.
Thanks,
Kai
Am 11.10.2013 16:30, schrieb Loyer, Jeff:
> I believe this (roughness) is a separate topic than surface finish.
> Surface finish is the addition of layers above the microstrip copper, it
> doesn't apply to stripline. Surface roughness is the roughness of the
> copper, it is an issue for both stripline and microstrip. Typically
> however, we consider the roughness of the top half of microstrip a moot
> point; I haven't seen any studies emphasizing that.
>
> Also, it's not a valid picture of the physics to represent the trace
> getting longer due to roughness.
>
> Jeff Loyer
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Randy Dawson
> Sent: Thursday, October 10, 2013 6:31 PM
> To: weirsi@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: surface finish
>
> One of the main reasons this comes up is in high frequency design, surface
> roughness can have a dramatic effect on RF performance due to skin effect.
> Some of the better 3D field solvers the surface roughness can be
> specified, and taken into account for calculated impedance and propagation
> delay (the trace gets 'longer' due to EM propagating closer to the
> surface).
> Randy
>
>> Date: Thu, 10 Oct 2013 17:00:14 -0700
>> From: weirsi@xxxxxxxxxx
>> To: si-list@xxxxxxxxxxxxx
>> Subject: [SI-LIST] Re: surface finish
>>
>> Welcome to the world of: "It depends."
>>
>> What is the product working environment? In particular: humidity,
>> temperature, and salts.
>> What are the product reliability requirements?
>> What is the product's longevity requirements?
>>
>> Steve.
>> On 10/10/2013 4:51 PM, Randy Norman wrote:
>>> Greetings
>>> I see a lot of discussions on surface finish. Losses and all can be a
>>> problem.
>>> Why is it necessary for traces?
>>> Some say you do not need it for traces because you have soldermask.
>>> Sure for the pads but what about everything else?
>>> Thanks
>>> Randy
>>>
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>>
>> --
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