Hi Werner!
     
     I agree with your assessment/description of the solderability 
     degradation for a thin solder finish but I think that you are giving 
     the Pb oxide too much credit as a contributing underlying mechanism. 
     Published data within the last 2 years has shown that Pb oxide is 
     present in only small volume/area in comparison to Sn oxide or CuxSnx 
     intermetallic oxide. The Sn and/or CuxSnx oxides dominate the 
     wettability causing the solderability problems with the increased 
     liquidus temperature issues of the Pb rich layer being a bigger 
     contributor than any Pb oxide issues. HASL finishes are definitely 
     headaches if good process controls are not followed and maintained.
     
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: Re: RE: IPC A 600 E:vert hasl
Author:  [log in to unmask] at ccmgw1
Date:    5/29/97 2:03 PM


Hi Frank, 
You responded to my message of:
     
>>Be careful about thin HASL solder layers; below 5 micro-meters (200 
>>micro-inches) solderability can decrease rapidly with shelf life. 
     
with a message dated 05/27/97 11:31:19, [log in to unmask] (frp) :
     
>I once read about a study in which the shelf life of 1 year was >simulated 
by a baking process. Then the grow of the intermetallic layer >Cu/PbSn was 
measured. The outcome of this investigation was that the >intermetallic 
layer increased by 0.8 microns. So to have a good >solderability after 
storage 1 year
>(normal conditions) you only need a minimum of 1 micron of PBSn.
>So I don't think that having less than 5 microns will give problems as 
>long as it is not below 1 micron.
>
>Kind regards
>Frank Petit
     
I was perhaps too brief in my response--I would wish things were quite that 
simple. I have clients that wound up with non-solderable soldering pads 
specing a minimum of 0.1 mils (2.5 microns) of HASL coating thickness, 
whereas specing 5 microns eliminated the problem. While IMC growth at low 
temperatures, like 20 C, are indeed slow, it rapidly increases with 
increasing temperature.  While it takes about 1 year to grow 0.8 microns of 
IMC at room temperature, the same thickness is obtained in about 3 weeks at 
40 C. 
It is also a matter of solder composition; the higher the Sn content the 
faster the IMC growth. 
Further, one has to consider what the HASL layer actually consists of. The 
copper pad is covered by (1) a thin layer of Cu3Sn IMC which in turn is 
covered by (2) a thicker Cu6Sn5 IMC layer, which is the layer that grows; 
adjacent to the Cu6Sn5 layer is (3) a layer from which the Sn has been 
depleted to form the IMC layers (this layer has a higher Liquidus temperature 
because of the higher lead content); this layer is (hopefully) covered with 
(4) a layer of the regular solder, which in turn is covered with (5) a layer 
containing non-solderable lead oxides. If the HASL process removes too much 
solder, the solder layer (4) becomes very thin or non-existent. The solder lay 
er (4) with its low Liquidus temperature is required during reflow soldering 
to break up the Pb oxide layer (5) as well to help liquify the Pb-rich layer 
(3). 
Colin Lea's book 'A Scientific guide to Surface Mount Technology is a good 
source for more detailed information.
Therefore, a total HASL layer thickness of 2 microns or less is sure to get 
you in trouble unless you have a perfect JIT capability. 
     
Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting 
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 201-543-2747
E-mail: [log in to unmask]
     
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