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For information on our Lead Free Products,
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Lead (Pb) has successfully been used in the manufacture of
electrical and electronic equipment for many years. Environmental interest on
the increasing number of discarded electronic products containing lead, that
may end up in landfill sites, have drawn attention to 'Lead Free' (Pb Free)
Soldering - even though electronics solders represent less than 2% of the lead
consumed annually.
Legislation - WEEE Directive 3rd Draft July 1999
In response to the concern, the EC has initiated a proposal
- the WEEE Directive (Waste from Electrical and Electronic Equipment).
Currently on the third draft, the proposal is principally concerning recycling
and aims at banning the use of lead from certain categories of electrical and
electronic equipment and PCB's by 2004.
' The objectives of
the Community's environment policy, as set out in article 174 (1) and (2)
of the EC Treaty, aim in particular at preserving, protecting and
improving human health and utilizing prudently and rationally natural
resources. Whereas this policy shall be based on the precautionary
principle, on the principles that preventative action should be taken,
that environmental damage should, as a priority be rectified at source and
that the polluter should pay'. Source - WEEE Directive (3rd Draft
Proposal).
A fundamental objective of the directive, is the control of
waste from private households and states that the directive will not work
until this is addressed. This proposal, whilst our concern is the removal of
lead and the effects to the manufacturing process, has major implications on
the use of lead, mercury, cadmium, hexavalent, chromium, PBB and PBDE's.
The directive states, the number of materials supplied will
have to be cost implicated, not only in collection but in policing and
reporting. Electrical and electronic equipment must also be designed, in such
a way as to facilitate their repair - possibly to be upgraded, re-used,
disassembled and recycled.
'Member states shall ensure that producers provide
information yearly on the quantities of electrical and electronic equipment
put on the market within the Member States, both by numbers and by weight'.
Source - WEEE Directive (3rd Draft Proposal).
There is nothing new about 'Lead Free' soldering. Lead Free
alloys have successfully been used in the electronics industry for many years.
The semiconductor industry have used combinations of reflow, dipping
operations and hand soldering using the alloy 96S. Although higher
temperatures are required than currently used tin/lead solders (requiring some
processes to be modified), indications are, that joints made with proposed
replacements, are at least as good as those made with tin/lead alloys. Lead
Free alloys can provide reliability and mechanical strength.
For the majority of consumer electronics the
Tin/Silver/Copper (SnAgCu) or Tin/Silver (Sn/Ag) alloy will be used, but for
some, a combination of boards, components and solder will be necessary.
LEAD FREE ALLOYS
The table below shows the main Lead Free alloy combinations
available.
| Alloy |
Advantages |
Disadvantages |
| SnCu Tin/ Copper |
Cost effective, Available |
High melt point 227°C.
Poor
mechanical properties |
| SnAg Tin/Silver |
Available, reliability, good
mechanical properties, better solderability than tin/copper.
Eutectic 221°C |
Cost, melting temperature 221°C |
| SnAgCu Tin/Silver/Copper |
Reliability,
Solderability,
Melting temperature 217°C. Ideal for general purpose soldering |
Melting temperature 217°C |
| SnAgBi Tin/Silver/Bismuth |
Reliable. Melting
temperature 206°C, Solderabilty, wettability, strength |
Fillet lifting,
sensitivity to
lead contamination |
| SnZnBi Tin/Zinc/Bismuth |
Melting temperatures 189°C,
close to tin/lead eutectic |
Paste shelf life, needs
aggressive fluxes, excessive oxidation, corrosion. |
Source - DTI UK
'Specific alloys may be dropped into processes
without significant problems' - Source DTI
UK
Warton Metals manufacture a range of Lead Free alloys to
suit all applications. Please see Lead Free Selector Guide
Lead Free Wave Soldering
Optimization of Lead Free soldering processes are required
when wave soldering, to eliminate fillet lifting defect, (especially with
bismuth containing alloys and components with lead containing terminators).
Problems of fillet lifting and poor reliability occur with boards which have
initially been soldered using Lead Free technology, but which subsequent
soldering is repaired/ reworked using Lead based solders.
'I n the case of wave
soldering, the preferred sequence is to Pb Free boards and components
before Pb free solders, due to problems associated with fillet lifting' - Source
DTI UK
Fillet Lifting (Lift off phenomena), associated with wave
soldering, is essentially the parting of the solder fillet from the land
surrounding the hole in through-hole joints. The precise mechanism of its'
formation is not fully understood.
Warton Metals have customers currently using 96s for wave
soldering applications, independent of the lead free issue with great success.
Pot operating temperatures have been observed varying form 245 °
C to 255°
C depending on substrate.
Solder Paste
The lift of phenomena discussed previously, is mainly
associated with wave soldering applications, however this does not say that
problems will not be seen using lead free solder paste on tin/lead boards. It
is advisable to change PCB's to Lead Free, prior to running solder paste.
Using Wartons TSC alloy, little process change will be necessary, with peak
temperatures as low as 238 °
C (melting point 217°
C).
'Marconi Communications with experience of use of the
SnAgCu, have described the alloy a virtual 'drop in' replacement for reflow
soldering due to limited changes required to the soldering processes' - Source:
DTI UK.
Hand Soldering
The Tin/Copper alloy (99C) will probably be selected over
96S or TSC due to cost for hand soldering operations. Great care must be shown
here as the metallurgical structure off 99C results in soldered joints
changing over the course of time to a very soft structure, which could have
great implications on joint reliability and integrity. Warton offer a complete
range of Lead Free Alloys suitable for hand soldering operations.
Printed Circuit Boards
Lead Free HASL coatings are confirmed, with no or only
slight changes to the production process. Lead Free alloys perform on OSP
boards but have improved solderability on metallic coatings such as Tin/Silver
or Palladium. Board laminates CEM-3/FR2 are generally unsuitable for wave
soldering at 255 °
C.
Lead Free Board Coatings
Many Lead Free board coatings are available such as tin,
gold over nickel, silver palladium and organic coatings. Research on board
coatings performed by the ITRI found that tin coatings performed excellently
and were particularly compatible with lead free solders, retaining their
solderability after steam ageing and reflow. A silver coating proved most
compatible with tin/silver solder in the 'as received' and reflow cycled
conditions. Palladium achieved excellent wetting in tin/lead solder and
survived ageing conditions. Palladium was most compatible with tin/silver.
Nickel/gold coatings proved compatible with lead free solders in the 'as
received' condition but performed less well after ageing. Lead Free alloys can
be applied to HAL (Hot Air Leveling) boards.
Components
Components are traditionally 'tinned'. Tin/Lead plate or
dipped solderable coatings are applied onto the leads before the soldering
process. The coating is widely used throughout the industry to give good
wetting, while at the same time not affecting the composition and micro
structure of the solder joint. A concern with the continued use of Lead Free
alloys 'is fillet lifting'. There are problems with the changes from tin-lead
to lead free solder with the commonly used Nickel/Palladium alternative
finishes. Currently there is a lack of information on the reliability of
complex devices, such as BGA, mBGA's and flip chip attachments.
Aluminium Electrolytic Capacitors
Electrolytic capacitors and other sensitive components need
to be hand soldered after reflow. These have been shown to be incompatible
with higher temperature soldering required by most alloys. The normal maximum
operating temperature is 225 °
C.
Cleaning Processes and Fluxes
Fluxes required to chemically clean surfaces so a sound
joint can be achieved, will not require significant changes.
Important Facts -Drossing of Lead Free Solders
ITRI carried out tests to investigate the amount of dross
produced by lead free solders. Lead Free solders were found to behave
similarly to Sn-Pb40 in air. For all alloys, black powder particles consisting
of solder metal and oxide enveloped by oxide were produced. In the Sn-Pb40
dross, the solder easily wet the powder particles and a dark pasty dross of
intermixed solder and powder particle was seen. Lead free alloys produced
powder particles that were not easily wetted by the solder. Therefore dross
produced had less powder intermixed, giving it a more metallic appearance,
with the remaining powder unattached. This made dross easier to remove from
the solder bath. All alloys displayed a reduction under nitrogen. Source
- Lead Free Fact Sheet LFS988-Lead Free Drossing of Lead Free Solders.
Dissolution and intermetallic Growth.
ITRI estimated dissolution and intermetallic growth rates
for various coatings and substrate metals in molten tin/lead and Lead Free
alloys. The dissolution rate of silver was shown to be slower in lead-free
alloys initially containing the metal as an alloying element, than those in
tin/lead. Dissolution in non-silver containing tin/copper alloy was more
rapid. The dissolution rate of palladium in all lead free alloys tested was
lower than in tin/lead, whereas that of gold was higher.
The rate of copper dissolution in tin/copper was found to
be comparable with tin/lead, whilst dissolution in tin 3.5Ag was faster. The
dissolution rate of iron and nickel into all the solders tested, was very low.
Nickel dissolution rates in lead free alloys were faster
than in tin/ lead and it is possible that a 2 m
m barrier of nickel could dissolve in a lead free solder if left for an
extended time. At normal peak soldering temperatures / times and with the use
of extended electronics fluxes, this should not be the case.
Intermetallics were formed at the solder/substrate
interface. In all cases layer thickness on palladium was seen to be greater
with tin/lead alloys than the lead free alloys, particularly at higher
temperatures, lead free alloys formed thicker layers with gold than tin/lead.
Visual Guidelines
The DTI has commented that new visual guidelines for Lead
Free joints will be necessary, as Lead Free joints appear different to the
Tin/Lead joints.
Process Of Change
|
Step 1 |
Source Lead Free
Substrates
i.e. Copper finish PCB's |
|
Step 2 |
Evaluate Lead Free Solder
Materials
i.e. Warton Solder Paste,
Bar Solder, Solder Wire. Please See Warton Lead Free Selector Guide |
|
Step 3 |
Quality & Reliability
Tests for product Performance |
|
Step 4 |
Start to source Lead Free
Components |
|
Step 5 |
By 2004, Ready To 'drop
in', replacement technology |
Conclusion
Lead elimination in not going to happen overnight. There
will be a transition period during which contract manufacturers and OEM's will
be able to refine their manufacturing processes. 'Drop in' alloy solutions are
already available, with the Tin/Silver/Copper alloy appearing to be very
attractive for general use. A combination of the correct board, components and
solder products will make the move to Lead Free easier.
Warton can provide technical help throughout the move to
Lead Free. For more information, a site visit or technical datasheets/samples,
please do not hesitate to contact Sales & Technical Enquiries on: 01706
218888 or E.mail sales@warton-metals.co.uk
Warton Metals offer a complete range of ‘Lead Free’
alloys and 'Lead Free' ready fluxes to suit all applications. All products are
manufactured in the UK to ISO 9002, to meet national and international
standards.
References
Lead Free Soldering - An analysis of the current status of Lead Free
soldering - DTI April 1999 URN 99/782
Website: http://www.dti.gov.uk
ITRI Ltd
Silver Migration & Lead Free Soldering Fact Sheet LFS 989 - ITRI
(International Tin Research).
Drossing of Lead Free Solders - Lead Free Fact Sheet LFS988 ITRI.
Dissolution and Intermatallic Growth Rates in Lead Free Solders. LFS9814
ITRI
Website: http://www.itri.co.uk
WEEE Directive - (2nd Draft Proposal) - European Parliament and
Council Directive on Waste Electrical and Electronic Equipment amending
directive 76/769/EEC 7/98
WEEE Directive - (3rd Draft Proposal) - European Parliament and
Council Directive on Waste Electrical and Electronic Equipment amending
directive 7/99.
Website: http://www.icer.org.uk
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