I have seen the "perfect mouse bite" condition a number of times. The bite is perfectly round and on one side of the trace. The most common cause is "hydrogen overvoltage". I learned the term from AES (American Electroplaters Society). It is a condition where small hydrogen bubbles are produced at the edge of the traces or around pads preventing plating. The bubbles are caused by high voltage plating. Understand that many platers (bumper platers) regulate the power going to the electrolytic bath by adjusting voltage. The PWB industry controls power by controling amperage. The PWB practice is to calculate the amps per square foot (ASF)for each job. Controlled either way, if the current is too high you get bubbles, as water is broken down by a process call hydrolization. Oxygen bubbles are formed on the anodes and hydrogen bubbles on the cathodes (PWB). I have never seen mouse bites where air aggitation create the condition. Aggitation by air, vibration and through hole aggitation removes the bubbles.
The bubbles are trapped on edges of traces that point "up" in the bath at the resist edge. Both side of the board may have the "bites" orented in the same direction. The bubble acts like spots of resist stoping plating under the bubble creating a small crater. If this occurs in the copper bath it will present as neckdowns on the edges of traces. If the bubble presents in the tin/lead process is there will be no tin/lead resist in spots at etching and the results if a perfectly round etchout.
The cause of the "hydrogen overvoltage" can by plating current being too high or a breakdown of the brightener system. High current is the most common cause. Uneven current distrubution due to racking, bussing and flight bar problems is a common cause when one cell of a bath produces the problem but another does not. Errors in measuring or estamating plating surface area is suspect if the condition is related to a specific part number. Rectifications issues are also implcated particularly if the fabriator is new to pulse plating or periodic reverse plating. You also have all the factors contibuting to the brighter maintenance to consider including calculating adds, shelf life, carbon treatment, contaminates, etc.
Other similar problems may be caused by severly nodular copper plating, lifted resist in a print and etch process, resist contaminated sprays depositiing on developed borads or layers, but these causes present themselfs with random distribution on both side of the trace or pad.
Sincerely,
Paul Reid
Program Coordinator
PWB Interconnect Solutions Inc.
235 Stafford Rd., West, Unit 103
Nepean, Ontario
Canada, K2H 9C1
613 596 4244 ext. 229
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Scott Decker
Sent: Thursday, February 23, 2006 2:34 PM
To: [log in to unmask]
Subject: [TN] Board etching question for you board shops out there.
All you board gurus out there, I have a question for you.
What are the reason(s) I see traces that look like a mouse has
nibbled on the copper to etch it, as opposed to acid etching the copper?
I'm looking at some boards that look like the resist was put on by a
drunk with an old marker pen! The vias are not even round and the SMD
lands have ragged edges also. This shop is off my list from here on out
and going to be rebuilding some boards due to minimum trace problems
too, but can anyone tell me what causes the ragged edges like that? I
understand that specs are looking at averages, which means a high and
low end of the spectrum. I think I know who put the low numbers into the
pot. :-(
Thanks for any info.
Scott Decker
AKA: Padmasterson
Broad Reach Engineering CAE
Ph. 480-377-0400 Ext. 34
FAX. 480-968-4597
Tempe, AZ.
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