I believe you are just experiencing shadowing.  This is allowed as conformant etchback.
Even if it isn't shadowing, remember you can use the shadowing rule to get two chances at effective etchback.
Chris

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Uppina Nagaraj
Sent: Monday, August 08, 2011 11:34 PM
To: [log in to unmask]
Subject: [TN] Query on Plasma Desmearing

Hi All,

Further to my query sent previously, I got several responses, based on which we had verified the gas flow meters and RF power and they were found to be ok.

However, the results are still inconsistent.
I've posted some photos in the link below....

http://ipc-technet.groupsite.com/gallery/20924

In this trial we had purposely increased the gas flow and the timing. The drilled hole wall is very uneven due to plasma etchback. However, it can be seen that near the IL Copper the epoxy is not "etched back".
This phenomenon is randomly seen (no specific trend).

I don't know how significantly the temperature inside the plasma chamber affects the performance of plasma.

Any valuable inputs on the suspected cause and possible solutions would be of great help.

Thanks & Regards,
U.Nagaraj


---------------------------- Original Message ----------------------------
Subject: Query on Plasma Desmearing
From:    [log in to unmask]
Date:    Thu, July 28, 2011 10:32 pm
To:      [log in to unmask]
--------------------------------------------------------------------------

Hi All,

We are manufacturers of bare PCBs and as a part of PCB processing we have Plasma desmearing step.

Recently we're having some problem in getting etchback in PTH consistently. We have a plasma desmearing machine, where the loading of the panels is done horizontally.
The plasma desmearing is done in 2 cycle steps...
a) First cycle is done by purging O2
b) Second cycle is done by purging O2 & CF4

The volume of the gas and the duration of desmearing is set based on the material type and the thickness of the PCB.

Our processing parameters are set since quite some time and we didn't had any issues in getting etchback consistently. We've not changed any process parameters and/or the supplier source.

We are checking the level of desmearing by conducting desmearing on bare epoxy sheets and checking the weight loss. The results of the weight loss are also not consistent. The results are similar in the PCBs  also, the etchback is not consistent.

It would of great help if anyone can share the possible causes of the problem and suggest remedies for the same.

Thanks in advance!

Best Regards,
U.Nagaraj



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