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May 2003

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Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 13 May 2003 12:36:59 EDT
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Hi,
Part of the confusion results from sloppy language.
(1) the settings of reflow machines are reflow soldering recipes, not reflow 
soldering profiles. Different reflow soldering profiles will result from the 
same recipe at different locations on an PCA and will depend on the thermal 
masses and access of the heat transfer media involved.
(2) solder "wetting" is not just getting the solder liquid and touching the 
surface to be "wetted;" wetting includes all of that,  but also the 
dissolution of the base metal [different base metals require rather different 
amounts of heat because of thier wastly different dissolution rates in tin] 
to form intermetallic compounds (IMCs) which give you the desired 
metallurgical bond.
Chris,
ENIG boards require higher temperatures than HASL boards. 
When you solder to a HASL PCA, you are soldering to an already existing 
solder joint with the IMC in place and all you have to do is melt the solder 
volumes. Given todays reflow machines and belt speeds, the peak reflow 
temperature for the slowest heating solder joint should be Liquidus 
temperature for the solder used plus 20°C [thus, for 63Sn/37Pb the minimum 
peak reflow temperature is 203°C and should be held for about 5 sec to assure 
consistent high quality solder joints to copper with OSP.
When soldering to nickel, you need Liquidus temperature for the solder used 
plus 35 to 40°C [thus, for 63Sn/37Pb the minimum peak reflow temperature is 
218 to 223°C, because Ni dissolves so much slower in Sn than Cu. You can 
easily test whether you have achieved proper solder joints--prying a solder 
component off [chisel or screw driver test], will result in the soldering 
pads coming off the PCB or component or the SJs fractuing in the solder 
volume for stress-concentration geometries. If the separation results in the 
solder and the soldering pads separating at the interface, no metallurgical 
bond has been formed either because of insufficient heat or sme solderability 
deficiency.
The Au on the Ni protects the underlying Ni from oxydation, and actually 
promotes a slightly faster spreading of the liquid solder over the Ni surface 
because of the high dissolution rate of Au in Sn.
Mike's advice makes practical sense; it presupposes that you actually measure 
solder reflow profiles --as you should so you know what they really are.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com 

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