Hot air solder leveling in the lead-free era
Hot air solder leveling
in the lead-free era
by Keith Sweatman, Nihon Superior Co., Ltd., Osaka, Japan
introduction
that solder at its interface with the copper.
Although the advantages of
The wisdom traditionally expressed by ad-
Because, when a tin-containing solder
hot air solder leveling (HASL)
vocates of the hot air solder leveled (HASL)
wets copper an intermetallic compound,
in providing the most robust
finish when the solder used was a tin-lead
Cu Sn , forms almost instantaneously, the
6 5
solderable finish for printed
alloy—‘nothing solders like solder’—remains
evidence that wetting has been achieved is
circuit boards are well recog-
equally valid in the lead-free era. A strong
provided by the presence of that interfacial
case can be made that when properly
nized, in the years leading up to
intermetallic compound (Figure 1). Thus a
applied, a HASL finish provides a printed
the implementation of the EU
short form characterization of the benefits
circuit board with the most robust guar-
of the HASL finish could be that ‘half the
RoHS Directive in July 2006,
antee of solderability currently available.
solder joint has already been made.’
the conventional wisdom was
However, the statement ‘nothing solders
The term used to describe the ease with
that it would have no place in like solder’ requires some expansion.
which solder, with the help of an appropri-
the new lead-free electronics
A fundamental requirement of a solder
ate flux, can wet a substrate is ‘solderabil-
manufacturing technology.
joint is metallurgical continuity between
ity.’ With wetting already established, the
By July 2006, however,
the copper pad of the printed circuit board
HASL finish delivers solderability un-
there were nearly 200 lead-free
and the termination of the component (Fig-
equalled by any of the alternative printed
HASL lines running in Europe,
ure 1). Components terminations can have
circuit board finishes that do not contain
and that number has continued
a wide variety of metallurgies that create is-
tin. The additional benefit not mentioned
sues that are beyond the scope of this paper
to increase since then. In the
in any of the foregoing characterizations
so that all that will be considered here is
more than 5 years in which
of the HASL finish is that when properly
that part of the solder joint that is the con-
applied it can preserve solderability for a
the lead-free HASL process
nection to the printed circuit board. For
very long period of time without the need
has been used in commercial
the current conventional technology that
for additional protection from the environ-
mass product, much has been pad is electrodeposited copper, so the short
ment or special storage conditions. There
learned about the operation of
form justification for a HASL finish could
are many anecdotal reports of printed
the process on the optimization
be stated more fully as ‘nothing solders like
circuit boards with a HASL finish solder-
of results. In this paper the
copper that has been pre-wetted with solder.’
ing easily after many years of uncontrolled
author will report on current
That statement of the key advantage of
storage
1
.
best practice on the operation
HASL is more technically correct because
of lead-free HASL lines and the
what this finish delivers is not so much
The place of hAsl in pcB finishing
the solder that is visible but the wetted
properties that can be expected
Until the electronics industry was forced
interface that lies beneath the surface of
of a properly applied lead-free
by the EU RoHS Directive
2
to consider
the prospect of eliminating lead tin-lead
HASL finish.
eutectic solder (nominally Sn-37Pb) ap-
plied by the HASL process was the most
widely used (in terms of the area of panels
Keywords: HASL, Lead-Free processed) printed circuit board finish
in North America and Europe
3
. HASL
was also widely used in Asia, but the
market share was smaller because of the
dominance in those markets of consumer
electronics. For the consumer electronics
industry, the short time between manu-
facture and soldering that resulted from
the effective application of ‘just in time’
inventory management systems meant that
the solderability of copper pads could be
satisfactorily ensured with an OSP finish.
Figure 1. The essential features of a solder joint that In the period leading up to the
This paper was originally presented at the IPC
provide metallurgical continuity.
implementation of the RoHS Directive
APEX Expo 2009 in Las Vegas, Nevada.
10 – Global SMT & Packaging – March 2009 www.globalsmt.net
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