1)Why
adopt lead-free process:
Lead is considered a toxic heavy metals. Once
in the body, It can arose toxicosis if excessive
and a little also can affect nervous and reproductive
system disorders. In recent years, Global
electronic SMT consumed solder up to 60000
tons and appear the trend of increase, all
of which lead to industry dreg influence the
environment very serious especially contain
lead salt . Now the whole world are concern
about reducing lead usage, especially, Many
large company in Europe and Japan apply them
mind to develop lead free instead of alloy,
and also have a project to reduce lead Usage
step by step from 2002 and downright clear
up in 2004.(the component of conventional
solder is 63SN/37pb is widely used by electronic
assembly in the present time )
2)The conditions
of lead free substitute:
1. Price: Although the cost of lead free substitute
is higher by 35% than 63Sn/37Pn at present
time and Many factories want the price have
not any change.
2. Melting point: most factories request immovable
temperature at least 150.C to meet work request
of electronic equipment, and as for the solving
, the temperature is different according to
the different application .
Welding rod of wave soldering: solving point
temperature should less than 345.C the working
temperature of brand iron .
Solder cream : Melting point temperature less
than 250.C
3. Electric features.
4. Lead heat nature is good.
5. The range of solid and liquid :In order
to form a good solder point , Most experts
suggest the temperature should limit in 10.C
for if the temperature of solderability alloy
too high the solder point may be break so
lead to the electronic products damage early.
6. Toxicity is low: the component of alloy
include no poison.
7. Excellent Wetting features .
8. Excellent physics features such as tensile
strength, elongation and fatigue. The metal
alloy have the strength and reliability of
Sn63/ Pb37 and left no prominent angle weld
in qualified products.
9. Repeatability of manufacture and the consistency
of solder: Electronic assembly craftworks
is a large batch manufacture and request it's
repeatability and consistency very high .If
some alloy components can't manufacture repetition
and melting point change great in large batch
manufacture that make the alloy exclude the
lead free substitute.
10. The appearance of solder: The appearance
of solder is similar to Tin/lead solder
11. Supply ability:
12. Compatible with lead: it's a long period
to transform in lead free system so lead still
used in the weld plate of PCB and the termination
of component.
3)At present
time several usual lead free substitutes:
1. The alloy used in solder cream: The melting
point temperature of 96,5Sn/3,5Ag is 221.C
and 95,5Sn/4,0Ag/0,5Cu is 227.C
2. The alloy solder used in wave solder and
handmade solder: The melting point temperature
of 99,3Sa/0,7Cu is 227.C. Concerning about
materials, although several new pattern solder
cream has already appeared in market and have
few effect. Even more the new pattern's melting
point temperature is higher than traditional,
so they still can't instead of Tin/Lead solder
at present time and still want to do some
craftworks adjustment.
4) The temperature
curve of reflower:
1. Higher melting point temperature quickly
reduce the craft window. Although the melting
point temperature of Tin/Lead and it's completely
solving temperature respectively 183.C and
205.C-215.C, even more the highest temperature
of PCB is 230.C-240.C, so craft left temperature
is about 15.C-35.C; As the highest temperature
of PCB has not any change, the melting point
temperature of usual Lead free solder and
its completely solving temperature respectively
is 217.C-220.C and 225.C-235.C, above of which
make the craft's temperature reduce to 5.C-15.C,
also the left craft's temperature request
high repeat precision of reflower stove in
order to indicate the detailed difference
of temperature of PCB.
2. Lengthen the time of fluidity: the fluidity
time of Traditional lead cream commonly is
40s-60s and the lead free cream is 60s-90s.
Following is the difference between the lead
free reflower and lead cream:
Figure(1) is the typical curve chart of
lead cream
From the figure(2),we can conclude: if the
fluidity time and temperature strictly limited,
the temperature will reach to 260.C and
damage the components of PCB
Solution method:
(1) keep Flux preheating temperature fixedness
before solving tin.
(2) Use more than 2 heat up zone as jointing
zone
(3) Reduce Each temperature's dimension
and add heat zone in order to available
craft adjustment.
(4) Shorten heat zone's dimension in the
same throughput to reduce oxidation.
(5) Recommend everybody to use Nitrogen
protection crafts but it's not necessary.
(6) Design new middle support setting to
reduce its distribution warp
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