Hot air solder leveling in the lead-free era
!
!
High Contact Angle
Dewetted Area
Figure 20. Dewetting of pad where there was insuf-
ficient thickness of solder over the intermetallic layer.
• While the vertical HASL process is
still the most widely used, the hori-
zontal HASL process provides the
most uniform finish and the high
throughput required to meet the
growing demand for this process.
references
Figure 19. Whisker growth on hot dipped and stressed tin-lead and lead-free solder coatings on copper.
1. Bob Willis, “Hot air solder levelling- is
it a HASL?”, Global SMT & Packag-
ing, September, 2008, pp4-6
author’s company, has found that whiskers factor in a HASL finish is the volume of
2. “Directive on the restriction of the
appear on a such a hot-dipped lead-free solder rather than its weight. The thin-
use of certain hazardous substances in
finish on copper only in areas where com- ner average thickness of the coating also
electrical and electronic equipment”
pressive stress was deliberately introduced, results in a measurable reduction on the
2002/95/EC
and then only after 2000 hours under the quantity of solder required per unit of area
3. IPC- private communication
most severe conditions provided in that processed.
4. B Toleno, “PCB surface finish options
specification—the combination of elevated
for lead-free manufacturing” EMS Now
temperature (60°C) and high humidity summary
February 23, 2005
(87%RH). It was noted that for the silver- • Lead-free HASL provides a printed
5. R Schueller, “Creep corrosion on
free SnCuNiGe alloy, whisker growth circuit board with a corrosion
lead-free printed circuit boards in high
ceased once the induced compressive stress resistant finish that can assure
sulphur environments”, .Proceedings
had been relieved, but in the SAC305 fin- solderability over long periods of
of SMTA International, 2007
ish whisker growth continued, presumably uncontrolled storage and through
6. K. Nogita, T. Ventura, C. M. Gourlay,
driven by the stress generated by ongoing multiple lead-free reflow profiles.
T. Nishimura and A. K. Dahle, “So-
corrosion in the hot humid environment • The lead-free HASL finish is su-
lidification characteristics of eutectic
(Figure 19). perior to the tin-lead HASL finish
Sn-0.7mass%Cu solder with trace Ni”,
in terms of uniformity of coating
Proc. of Japan Institute of Metals An-
Trouble-shooting lead-free hAsl thickness.
nual Meeting, Chiba, Japan, 27th-29th
solderability • The main quality requirement for
March, (2007) p.301.
The lead-free HASL finish is intrinsically the achievement of the full poten-
7. J Fellman, “Lead Free Solder From The
highly solderable, so if solderability prob- tial of the lead-free HASL finish is
HASL Process Lead Free Solder From
lems are encountered during assembly, the a coating thickness that provides
The HASL Process-The Newest Surface
first step in diagnosis is to eliminate other full coverage of the intermetallic
Finish Alternative The Newest Surface
possible causes of such problems, e.g. deg- layer that forms on a properly wet-
Finish Alternative” IPC/JEDEC 8th
radation of the activity of a solder paste. If ted copper surface.
International Conference, Lead Free
the lead-free HASL finish is identified as • Best results in the HASL process
Electronic Components and Assem-
the only possible cause, the problem will are obtained with lead-free solders
blies, San Jose, CA April 20, 2005
be that complete solder coverage was not that come as close as possible to
achieved on all areas to be soldered (Figure matching the eutectic behavior of
20). This thin coating can be the cause of tin-lead solder.
Keith Sweatman, senior technical advisor to
the dewetting apparent in Figure 9. • A lead-free alloy formulated for
Nihon Superior, is a graduate in metallurgical
minimum copper dissolution
engineering and began his involvement with sol-
The cost of lead-free hAsl makes control of the solder bath
dering technology with the International Tin Re-
Because of their higher tin content and, in composition easier and reduces the
search Institute, an organisation that did much
some cases, silver content, lead-free solders extent to which board reliability is
of the work that established a scientific basis for
are more expensive on a per-kilogram basis compromised by reduced copper
what was previously the art of soldering. He is a
than tin-lead solder. However, other sav- thickness.
corresponding member of several IPC standards
ings mean that lead-free HASL is not much • A lead-free alloy formulated for
committees relating to soldering technology and
more expensive than tin-lead HASL when minimum growth of the inter-
is a member of the technical committee of the
all costs are taken into account
7
. The sav- metallic layer ensures maximum
IPC Solder Products Value Council.
ings include the lower density of lead-free solderable shelf live and best final
solders, which provides an approximately solderability in multi-stage solder-
12% savings given that the important ing.
18 – Global SMT & Packaging – March 2009
www.globalsmt.net
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