Hi Earl and Michael,
One problem that your message makes obvious is, that I never saw the 'original
Afri question.' I responded to your statement that seemed to refer to all
soldering operations. Thus, my statements have been consistent.
Earl you say:
>Something I am very curious about and, obviously, have little knowledge of,
is
>HASL solder coating thickness. I always was informed the process, because of
>the leveling characteristics no matter how many times repeated, didn't add
more >thickness as the process kept it relatively inconsistent.
No, you can not add more solder in a HASL process, only further reduce
thickness—but it is much preferred to adjust the process to give you the right
thickness in one pass. The difficulty of doing this consistently is one of the
drivers for alternate solderable finishes, like OSP. Except for reducing
solder thickness, I do not see any good reason—but a number of bad reasons
(see also e-mail from 'asingh'—to repeat a HASL step.
Micheal you write:
>I'm almost left with the impression that Fused Solder provides a more
solderable >finish, while I thought that HASL was the preferred finish because
of its high >coplanarity, improving the ability to achieve high quality solder
joints on SMT or >mixed through-hole/SMT assemblies.
A HASL finish is a fused solder finish, as opposed to a plated tin/lead
finish. Plated tin/lead neither proves that the underlying surface is
solderable (you can plate onto oxydized copper) nor does it preserve
solderability very well because the plated layer is porous. Thus the need for
fusing both to demonstrate solderability and to preserve it. Just fusing the
plated tin/lead into solder results in a significant convex meniscus on top of
the soldering pads which can make component placement very difficult; thus,
the need for Hot Air Solder Leveling (HASL) where a jet of hot gas blows
excess solder off the pads.
The amount of solder blown off depends on the gas velocity, the temperature
reached by the solder, as well as on the size of the pad (both of which affect
surface tension); it is the last variable (not controllable by the processing
people) that provides an inherent variability of solder thickness left in the
given PCB code.
But even HASL'ed surfaces are not flat, towards the pad edges the solder
thickness goes towards zero—thus, HASL'ed pads will have reduced solderability
around their edges.
But the edges are not that important, it is the area away from the edges that
counts most. Data from Colin LEA ("A Scientific Guide to Surface Mount
Technology"—an excellent book) show that HASL solder thicknesses of 2 to 3
micro-meters and less have a limited shelf-life that also depends on storage
conditions, of course. For thin HASL solder layers, the amount of solder that
reaches low viscosity in subsequent soldering processes is smaller; if this
layer only reaches a higher viscosity pasty stage between the Solidus and
Liquidus temperatures because too much tin has been consumed in the formation
of the intermetalic compounds (IMCs) and the solder composition is lead-rich,
the lead-oxide surface layer may not be broken up sufficiently to be swept
away by the overlying liquifying solder paste and proper wetting may not
occur.
Unless grossly violating J-STD-001 specifications, adequately wetted solder
joints are good quality solder joints (for Class 3 some spec restrictions have
actually been shown by a German study to result in somewhat lesser
reliability). However, good quality solder joints may still lack long-term
reliability because the design may be inadequate and result in large thermal
expansion differences during storage, shipping, and operational use.
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]
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