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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