On Wed, 24 Apr 1996 17:07:53 -0700 [log in to unmask] 
wrote:

> From: [log in to unmask]> Date: Wed, 24 Apr 1996 17:07:53 
-0700
> Subject: gold plating problem
> To: [log in to unmask]
> 
>      Our Process
>      Isolated tabs in the center of the board to be gold plated are defined 
>      by a primary layer of resist.  After copper and tin-lead plating them, 
>      we vacuum laminate 3 mil dry film resist.(over the plated tin-lead and 
>      over the primary layer or resist)  This top layer of resist is 
>      selectively imaged to produce an opening exposing the area to be gold 
>      plated (which now has tin-lead on it). Then the tin-lead is stripped, 
>      and the nickel and gold are plated using our usual sulfamate 
>      nickel/soft gold process. 
>      
>      The Problem
>      The plated nickel peels from the copper surface.  The peeling is 
>      always near traces connecting to the tabs.
>      Therefore I suspect that tin-lead stripper is being trapped under the 
>      top layer of resist and coming out during plating. 
>      
>      Solutions?
>      I don't suspect the nickel/gold process is the problem, as the usual 
>      yield on products without this tin-lead stripping step is nearly 100%. 
>       
>      We have no option to change the design of the board to facilitate 
>      manufacturing. ie: No bussing of the tabs is possible therefore base 
>      copper must be used as a conductive path prior to etching.
>      
>      We've tried: 140 degree F water rinse after stripping, conveyorized 
>      nitric tin-lead stripper, immersion and conveyorized peroxide tin-lead 
>      strip, a second strip cycle, UV bump (160mJ), among other things.
>      
>      Any help would be appreciated.
> 
									
									
									
									
	Peter:I agree with you that the Sn/Pb stripper is being trapped.The 
problem is even if you can get the stripper rinsed out the chemistry from 
each bath in your Ni/Au plate line is also being trapped and must be rinsed 
out so it doesn't leach out in the next process tank.This may require better 
rinsing at all the process steps.					
									
	What to do:							
		1.Verify that the Ni/Au line is functioning properly.Take one 
of the problem boards,do not put the second layer of on.Run this thru your 
Sn/Pb stripper(I'd use the conv.nitric)then thru the Au line.Do a tape 
test,if there are no problems this would then point strongly to your resist 
interphase theory.							
		2.Determine if the problem is the conformance of the top 
layer of resist to the circuit and the bottom resist.If that is what it's 
determined to be go back to the second lamination process.I would look at the 
exit temperature,conveyor speed and roll pressure to try and optimize the 
conformance and adhesion.						
		3.Check to see that the entrapment is not being caused by the 
Sn/Pb stipping solution working it's way under the first layer of resist and 
starting to partially strip the Sn/Pb underneath creating pockets and gaps.If 
this is happening try to minimize the amount of time in the Sn/Pb strip.	
		4.You may want to consider using a two mil film instead of 
the three mil for the second resist.Contrary to popular belief we have seen 
better conformance with the two mil.					
		5.Another way around the entire problem would be to do a 
Cu/Ni/Au first plate for the Au features,strip the resist then come back with 
the primary image for Cu-Sn/PB plate with the Au image masked out.	
									
		If it makes you feel any better you are not alone in fighting 
this problem using this style of construction.GOOD LUCK!		
									
									
					Regards 			
					Michael Barmuta			
					Staff Engineer			
					Fluke Corp.			
					Everett Wa.			
					206 356-6076