Promoting the commercial adoption of lead-free solder and ...

Promoting the commercial adoption of lead-free solder and evaluating its reliability
Report 2
Promoting the commercial adoption of lead-free solder and evaluating its reliability
Hirokazu Tanaka Technical Development Headquarters
Akihiko Saito Measurement and Test System Business Division
Toshiro Nagayama Measurement and Test System Business Division
Hiromi Umeda Quality Assurance Department
Since April 2002, we at Espec have been mass producing PCBs with lead-free solder. Leadfree
solder has different characteristics than the leaded eutectic solder (Sn-Pb) that has been
in use for so many years, and so this change has required an analysis of how to revamp the
mounting process as well as a reappraisal of the facilities and design for mass production. A
wide variety of electronic parts are mounted using lead-free solder, creating the problem of how
to evaluate PCBs for reliability in actual products regarding thermal stress to the parts resulting
from the higher mounting temperature that is required.
This report will present the details of our efforts to promote the adoption of commercially viable
PCBs using lead-free solder as well as the results of our reliability test evaluations of PCBs in
mass produced products.
1 Introduction
Solder has played a major role as joint material for the electronics industry, but the lead
(pb) constituent of solder has become a burden on the environment. 1),2) The EU (European
Union) response to this problem has been to put the RoHS (Restriction on the Use of Certain
Hazardous Substances) 3) into effect from July 1, 2006, and worldwide, a robust movement
for the commercial adoption of lead-free solder is now under way.
Pressing forward on the commercial adoption of lead-free solder, JEITA (the Japan
Electronics and Information Technology Industries Association) and NEMI (the U.S. National
Electronics Manufacturing Initiative) and Soldertec (an international center, located in the
U.K., for tin research and dissemination of lead-free solder information) have collaborated in
drafting the "Framework of the World Lead-free Soldering Roadmap," which has been hailed
as a milestone in the move to adopt lead-free solder. 4) All associated manufacturers are now
promoting the development and adoption of lead-free solder. 5)
We at Espec have been promoting the adoption of lead-free solder since 1999, and we
initiated mass production of PCBs with lead-free solder for use in our own products from
April 2002 with the aim of reducing the burden on the environment.
Table 1 shows the details of our efforts working for the adoption of lead-free solder. Since
1999, we at Espec have been working toward the commercial adoption of lead-free solder
through establishing a "committee for the promotion of lead-free solder" and a "committee
for the adoption of lead-free solder." Our related departments (e.g., design, manufacturing,
and quality control) are working closely together to study the feasibility of adopting lead-free
solder.
ESPEC Technology Report No.17 10

Promoting the commercial adoption of lead-free solder and evaluating its reliability
2 Efforts to adopt PCBs compatible with lead-free solder
Table 2 shows the details of the efforts we have made for the adoption of lead-free solder.
The results of our investigations into processes and materials for parts, design, and
manufacturing are presented below.
Design
Manufacturing
Parts
Table 2 Details of efforts for the adoption of lead-free solder
Processes and materials Details of efforts
PCBs
Metal masks
Reflow soldering
process
Flow soldering
process
Manual soldering
process
Abolished solder finish (hot-air solder leveling). Instituted
pre-flux (OSP).
Reappraised and improved circuit land pattern size.
Added PCB identifying marking (lead-free label) (Fig.1-a).
Improved screen aperture.
Improved position of printed circuits, hole precision, and
precision of mounting position.
Made trapezoidal profile with temperature control inside
reflow oven (Fig.1-b).
Controlled peak temperature (max. 240℃).
Made temperature distribution uniform.
Reappraised solder paste shelf life and useful life after
opening.
Ensured complete through-hole wetting (strengthened
pre-heat control).
Instituted measures preventing temperature drop during
solder injection.
Modified nozzle discharge tip (extended immersion time).
Took countermeasures to dross (solder oxides)
Reappraised control standards for solder constituents
(adjusted to min. Pb = 0.5%, Cu = 0.9%)
Reappraised soldering training standards and retrained.
Reserved special tools and special work area.
Prohibited use of cut solder wire.
Modified temperature of soldering gun (from 350℃ to 370
or 380℃).
Reconfirmed heat resistance temperature.
Confirmed surface finish materials (plating) of parts.
For the "design" of PCBs, the conventional solder finish (hot-air solder leveling) was
abolished to eliminate lead contamination, and the copper pattern and pre-flux (OSP,
Organic Solderability Preservative) were consolidated. An identifying mark (Fig.1-a) was
added to the lead-free solder PCBs to aid in recycling and in service inspections of the
mounting PCBs.
ESPEC Technology Report No.17 12

Promoting the commercial adoption of lead-free solder and evaluating its reliability
15
Evaluation solder
materials
PCB materials
Temperature cycle
test
High temperature
test
High-temperature,
high-humidity test
Mixed gas test
Field reliability
testing
Evaluation items
Table 3 Evaluation materials and conditions
Lead-free solder: Sn-Ag-Cu
Leaded solder: Sn-Pb eutectic solder
Glass epoxy (FR-4, t = 1.6 mm)
-25℃←→+80℃
Duration: 30 min. each temperature (1 hour/cycle)
1000 (3000) cycles
+80℃, 1000 hours
+80℃, 90%, 1000 hours
+25℃, 75%, H2S = 0.01 ppm, SO2 = 0.2 ppm
NO2 = 0.2 ppm, Cl2 = 0.01 ppm, 500 hours
Installation equipment: Temperature and humidity chamber
(Model name: Platinous Series)
Facilities sites: Espec Utsunomiya Technocomplex,
Fukuchiyama Plant
Check operation (e.g., input and output, temperature and
humidity characteristics).
Measure power consumed, measure shear strength.
Observe appearance and cross sections.
(a) Portion of evaluation PCBs (b) Field reliability testing conditions
Photo 1 Evaluation PCBs and test conditions
ESPEC Technology Report No.17

From the results of the reliability tests showing no failure of the lead-free product PCBs
even after 3000 cycles, for estimated life in worst case condition C we get the following.
3000 cycles x 4.5 (acceleration constant) = 13,500 cycles
13,500 / (twice x 365 days) = approximately 18.5 years
This provides an estimated life of 18.5 years at field conditions. However, in estimated life,
we must allow for a safety coefficient taking into account the degree of product completion,
solder usage environment settings, and different usage conditions. In the future, we plant to
verify these estimates using the results of continuous field reliability testing.
Accelerated test
conditions
Field conditions
A
B
C
4 Conclusion
Promoting the commercial adoption of lead-free solder and evaluating its reliability
Table 4 Acceleration coefficient estimates using
Manson-Coffin's modified formula
Minimum
temperature
-25℃
+25℃
+35℃
+25℃
Maximum
temperature
+80℃
+50℃
+60℃
+60℃
△T
(temperature width)
24 cycles/day
2 cycles/day
Acceleration constant
(1/AF) for accelerated
testing using field
conditions
For this report, we obtained results of reliability evaluations from our efforts to adopt
commercially viable PCBs using lead-free solder, and we arrived at the following
conclusions.
(1) By reappraising the manufacturing and design processes to adopt lead-free solder for
mass production, we found that it is possible to maintain mountability equivalent to or
better than that of conventional leaded solder.
However, for manual soldering processes, difficulty in improving workability remains
due to the characteristics of lead-free solder compared to conventional leaded solder.
(2) The higher mounting temperatures and the resulting thermal stress on parts when
adopting lead-free solder were confirmed not to pose any problems in regard to the
mounting conditions and reliability test evaluation items used in these experiments.
(3) Product PCBs using lead-free solder have exhibited the ability to withstand a minimum
of 3000 cycles in temperature cycle tests and a minimum of 20 thousand hours in field
reliability testing.
ESPEC Technology Report No.17 20
△105℃
△25℃
△25℃
△35℃
on/off
frequency
AF=1
1/9.7
1/8.5
1/4.5

Promoting the commercial adoption of lead-free solder and evaluating its reliability
21
From the results of our evaluations as seen above, we can assume that it is possible to
maintain reliability equivalent to or better than that of conventional leaded solder when
using lead-free solder on PCBs. We therefore conclude that it is possible to use lead-free
solder in mass production.
Based on these results, we at Espec intend to continue promoting the adoption of lead-free
solder.
[Bibliography]
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Report, No.13, p.1-8, 2002
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3) The Council of European Union, "Directive of the European Parliament and of the Council
on the restriction of the use of certain hazardous substances in electrical and electronic
equipment" Official Journal of the European Union, Feb. 12, 2003
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ESPEC Technology Report No.17