Does the bare copper (before soldermask) fail with the one vendor's laminate 
 and not the after soldermask test?
 
 Here are a couple more things that can effect SIR. 
 
 
 The laminate from different vendors CAN effect the SIR values. Some of the 
 reasons are:
 
 1) The thickness of the resin on top of the glass weave is important. Too 
 little resin on top of the glass especially in a bifluoride solder strip, 
 (which will attack the resin) could yield areas where there is no resin 
 protecting the glass. The plating chemistries can now filter through the 
 exposed lateral glass bundles and cause SIR problems. The after soldermask 
 panel processed at the same time with the same material could actually be 
 higher than the bare copper values because half of the comb area is covered by 
 soldermask.
 
 2) Do the laminate Vendors use the same build and material for the SIR 
 laminate? A higher resin glass weave would be better than a low resin glass 
 weave. 
 
 Is the material difunctional, multifunctional, or tetrafunctional.
 
 3) Different laminate suppliers may have different copper foil suppliers. The 
 copper foil could have different amounts of chromate  or zinc.
 
 Siline can be found in and around the foil. This is a metal 
 chelator. Excessive amounts can result in the tin or tin/lead in the 
 solderstrip chemistry to attach to the siline.  
 
 What type of solder strip chemistry is used. (nitric or ammonium 
 bifluoride? In nitric the tin/lead is more soluble and in the bifluoride the 
 tin is more soluble.
 
 
 
 To help narrow down what could be the cause and the list is long. 
 
 Take a cross section of a known good and bad comb. Look at the conductor 
 widths, the glass structure the resin on top of the glass to see if any 
 noticeable differences can be found.
 
 Evaluate the comb areas of a good and bad board under a microscope.
 
 When doing the SIR testing, take readings before humidity and after humidity is
 applied. If the failure mechanism is metal spots the values should be similar. 
 If the failure is due to salts or organics the readings before humidity should 
 be much higher than after humidity is applied.
 
 Compare the SIR values to any Ionic Contamination testing that is done. A high 
 run for ionic testing may give a clue as to the nature of the problem.
 
 Ensure that the SIR panels are carefully handled and that they are processed 
 the same.
 
 Steve Zimmer
 
 Hadco 
 
 
 
 
 ______________________________ Reply Separator 
 _________________________________ Subject: Re: IONIC TESTING
 Author:  [log in to unmask] at SMTPLINK-HADCO Date:    7/10/97 08:48 AM
 
 
 Rudy, You are SO right !!  As we all discover at some point in our lives, "you 
 really do get what you pay for" ....  I'm having a difficult time trying to 
 justify to Purchasing that just becuase a laminate manufacturer or (worse yet) 
 distributor says his material meets say Mil-S-13949/4, that doesn't mean we can
 indiscriminately "drop" it into out processing line and not expect anything to 
 change!!  
 
 
 At 10:54 PM 7/9/97 -0400, you wrote:
 >In a message dated 97-07-09 05:27:25 EDT, you write: >
 >>Having seen lots of SIR data from OTHER facilities, the problem could be a 
 >>fault in the tester, or technique.  Did good old Bob retire around that 
 >time?
 >> Did water quality in the rinse system change?  Did the DI water unit 
 >>suddenly go south?  There are lots of factors that can cause poor SIR. > 
 Don't
 >>assume any one factor is the problem.  >
 >Sir Doctor Guru of SIR
 >
 >You run two pieces of laminate, one from vendor A, the other from vendor B, 
 >through the same process, and one passes easily, the other fails miserably, 
 >and you repeat this test, with 100% reproducability.... if you can make 
 >another deduction from this data, TechNet is awaiting your assessment....:-) 
 >>
 >>I do agree that no two laminates are the same.  Varying levels of porosity 
 >>and varying resin compositions can dramatically effect SIR, or at least 
 >their
 >>processing characteristics.
 >>
 >Sir Doctor:
 >
 >Most of the poor, overstressed PCB fabricators have only their vendors to 
 >(pick one) 1. turn to for assistance  2. blame, and having been in both 
 >positions more than once, it is clear to me that we all need to see what the 
 >options are.   I for one, and suspect many others, never considered that 
 >laminate could vary to this extent.   This was a great learning experience 
 >for me, and I was hoping to spread the benefits....
 >
 >Yours in science, and PCB fabrication: >
 >Rudy Sedlak
 >RD Chemical
 >
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