Give Doug Pauls at Contamination Studies Labs an email - they have 
     lots of experience in this type of process testing/verification. His 
     email address is [log in to unmask]
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]
     


______________________________ Reply Separator _________________________________
Subject: Via Entrapment and LPI
Author:  [log in to unmask] at ccmgw1
Date:    6/26/96 4:51 PM


Greetings Technet
     
I'm looking for a "best solution" for my board vias. Actually I have a 
favorite approach I want to prove good.  This involves fairly dense double 
sided SMT boards 0.065" thick with via holes drilled at 0.015" diameter. 
The via count is about 2000 to 4000 per board. The design was intended for 
dry film soldermask with tented vias, so via pad spacing from adjacent 
conductors is about 0.010" and vias are routinely located under SMT 
components. I have been dealing with some standard dry film problems such 
as mask failure during assembly and reduced availability in the 
marketplace.
     
I want to use LPI on this design and ensure corrosive entrapment in the 
vias does not affect reliability. My assembly fluxes are "leave on" and 
pose no threat. My concern is the aqueous flux at HASL.
     
Opening all vias to solder is undesirable on this design due to location of 
some vias.
     
I want to apply LPI to both sides of the via (with no screened plugging 
step). This will generally leave a small hole on both sides, on occasion, 
one side may be plugged. I have used this process on some boards with 
excellent results. No via failures were experienced. Dissection of vias on 
product 6 - 12 months old found via hole walls generally bright and no 
evidence of deterioration was found.
     
I am operating under the assumption that the HASL aqueous flux (boiling 
point of 112C) largely flashes off during HASL and the subsequent water 
wash is effective. SIR also indicates clean boards. I just ordered 
IPC-AC-62A "Aqueous Post Solder Cleaning Handbook" hoping to find support 
of this approach.
     
Does anyone have experience with this approach? I am also looking for a 
measurement method that would determine the level of HCL (flux acid) 
present inside the via.
     
Thanks
Nat Perkinson ([log in to unmask])
BroadBand Technologies