Re: [Homebrew_PCBs] updates on cupric chloride etching. part 2
2003-04-09 by Adam Seychell
Grant, There is some interesting reading inside the technical section on http://www.oxfordvue.com/ http://www.oxfordvue.com/TechInfo/copper_and_acid_normal.html they suggest the correct procedure for acid titrations. It so happens that you DO NOT use an indicator because its more accurate to rely on the initiation of tubidity change rather than color change on indicator. The CuCl2 + NaOH will form insoluble Cu(OH)2 once all the free HCl acid is consumed. Well that news makes it easier and cheaper for us. There is a lot to learning about this etchant and how to properly use it and maintain it. I've been talking to Phil Culpovich via email who I think is one of the main people at Oxford V.U.E., Inc. Below is a copy of his reply. -------------------------------- Adam, Since the production shops that use cupric must have a faster regeneration speed, they use a controller and oxidizer to speed and control the process. In your case, the oxygen method is sufficient and tends to be safer since you virtually can't overdo the oxidizer additions resulting in the release of chlorine gas. As to the differences in free acid levels, the range boils down to a relationship between the need for quality and speed vs. the wide ranging results coming from different etcher designs. As a rule of thumb for conveyorized spray etchers, higher acid results in lower quality. Zero acid works best for quality since there is no acid undercut. The cuprous that forms during etching under the nozzles is soluble in HCl so uncontrolled etching occurs between the nozzles on the top surface of panels in a converized etcher where free acid is present. The drawback for zero acid can be a loss of speed but this doesn't have to be the case. If you have a sufficient quantity of nozzles with higher flow rates at higher spray pressures, the acid speed increase disappears while still maintaining the highest quality and consistency. I will forward a copy of a comparison of speed between etchers illustrating this point that was done by Circuit Research Corp. separately from the TechNet since no attachments are allowed through the listserver. In any case, in a spray etcher, a higher free acid than 2N has no appreciable benefits and just causes an environmental, safety and quality mess. In the case of your dip tank, etch speed will tend to be slower anyway unless there is some agitation of the solution or the part etched. Free acid will help here on the speed, as you mentioned, because of the solubility of cuprous formed from etching while in HCl. There would be no discernable quality advantage between zero acid and any other acid Normality chosen since you are not controlling the direction of the etch in any way. Keeping the free acid between 0.1N-0.5N, as you say, should give you the best compromise between speed, cost, environmental concern and quality. If the dip tank provides you with sufficient quality and quantity for what you are etching, then I would say you have progressed as far as you need to to keep your project under control and costs down. As to using the chemical oxidizers for regeneration (chlorine gas, hydrogen peroxide and sodium chlorate), this reduces the regeneration time required from your current several hours down to less than 10 minutes for all the oxidizer chemistries and automated control methods. Feel free to e-mail me any time if you have more questions. Best regards, Phil Culpovich pculpovich@... www.oxfordvue.com -----Original Message----- From: TechNet [mailto:TechNet@...]On Behalf Of Adam Seychell Sent: Wednesday, April 09, 2003 2:00 AM To: TechNet@... Subject: Re: [TN] cupric chloride parameters Phil, You response tells me I am more or less on track with the chemistry. It amazes me the fact that free acid concentration can vary across such high range. It wouldn't be surprising then to hear conflicting arguments on what is the best level of acid to use in CuCl2 etching. What I hope to achieve is a 5 gallon immersion etching tank, for occasional etching of single sided prototype PCB's. What you describe about the slow regeneration with air bubbles is spot on to my observations. Yes, the regeneration rate via air bubbles alone is many times slower than the rate of copper being dissolved. Fortunately the tank will have limited use and so it may continue being regenerated for a long time (e.g several hours) after the PCB has finished etching. I believe the volume of solution should be sufficient to hold the limited amount of copper(I) produced during the relativly short etching period. To test when regeneration is complete, I take a few drops of etchant and add to 5 ml of water, if a cloudy precipitate forms then it indicates a significant amount of copper(I) is present and regeneration must continue. I have tested this on small scale, and it seems to give a good indiation, the time for regeneration can be up to 3 or 4 hours. I guess the industry have much cleaver more automated methods of keeping the solution regenerated with chemical oxidizers. Adam --------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8e To unsubscribe, send a message to LISTSERV@... with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to Listserv@...: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to Listserv@...: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/html/forum.htm for additional information, or contact Keach Sasamori at sasako@... or 847-509-9700 ext.5315 -----------------------------------------------------