The move to lead-free solder has
prompted two questions: what will replace tin/lead solder?
And, what effects will the replacement solder have on
manufacturing and test processes?
Replacement Solder Compounds
The jury is still out but two alternative
solder compounds seem to be emerging, a tin/copper/silver
solder and another made up of tin/copper/nickel. For
both compounds, the melting point is around 230/240°C,
considerably higher than the melting point for tin/lead
solders. Actual reflow temperatures peak at 220°C
for tin/lead and 260°C for tin/copper/silver. The
higher oven temperature required by the replacement
solders causes concern among board manufacturers and
board test engineers.
Higher soldering temperatures would
necessitate higher-temperature components on the boards
and high-temperature devices have shorter shelf lives.
Fortunately, tests so far on the higher temperature
soldering process do not indicate any internal damage
such as internal bond wires lifting, but manufacturers
are concerned about board warping, the effect of higher
temperatures on heat sinks which act as heat reservoirs,
tombstoning and ‘tin whiskers.’ When pure
tin bonds a device to a board, very thin strands or
whiskers of tin can spread out at the bond boundary.
These whiskers can eventually cause shorting to other
connections, either by migration or by breaking off.
Adding lead to tin reduces or removes the problem. For
more information on tin whiskers, click
here.
Silver can also causes problems for
electronic manufacturers. In a polluted environment
such as an industrial setting, sulphur and chlorine
compounds may be present. These react with silver to
form a friable (crumbly) layer on the silver. This layer
is most likely a silver chloride compound. It has no
effect on the conductivity of the silver but it is friable
and can break off the surface and contaminate an instrument
or electronic system. Because silver chloride is conductive,
it can cause shorts when it settles on or is close to
interconnects.
Possible effects of lead-free
solder on test
Lead-free solder will have several
effects on current test techniques. For example, the
appearance and geometry of lead-free solder joints will
be different from tin/lead solder joints. This may affect
manual inspection, AOI or AXI test techniques, although
at least one AXI supplier, Agilent Technologies, claims
that their 5DX AXI system will be affected very little
by lead-free solder.
Another concern arises during board
re-work where soldering irons must be at very high temperatures
of over 300°C to work with lead-free solder. These
high temperatures can damage components on the board
that is being re-worked. In addition, intermittent opens
can be caused by an oxidation that forms between the
pad on the board and the tin/copper/nickel solder.
Affecting ICT fixturing
Lead-free solder may also cause problems
for ICT test processes because the lead-free solders
are harder to penetrate with the tip of a nail probe.
At a time when most manufactures are trying to reduce
the force placed on printed circuit boards, more force
per nail may be needed to test lead-free solder joints.
This would add to the parts-per-million of ‘µStrain’
placed on the board by the ICT system, shortening the
useful life of the nail probe. Currently, nails probe
at 6 oz of force on cleaned boards or 8 oz of force
on non-cleaned boards. The figure for non-cleaned boards
with lead-free solder may have to rise to 10 oz but
there is considerable resistance within the industry
to anything greater than 8 oz of pressure. For more
information about board flex and µStrain requirements,
click here.
The outcome of this situation may be
the following:
- More robust and more costly, nails
may be required.
- Larger target test pads may be
needed for the more robust nails, taking up more,
not less, board real estate.
- Multi-prong nail tips could be
used but this would be more expensive than single
prong tips.
Boundary scan as an alternative
The transition to lead-free solder
could be facilitated by an increased use of JTAG test.
Because boundary scan does not require physical access,
it does not place any added strain or stress on the
board under test. And since it complements ICT processes,
any increase in the deployment of boundary scan would
reduce the number of test points and nail probes needed
for ICT procedures. Given the increasing expense of
nails that are strong and durable enough to test lead-free
solder joints, any reduction in the number of probes
deployed in an ICT fixture would reduce the cost of
test significantly.
Summary: The short-term effects
of lead-free solder on board test |
Lead-free
solder may shift test methods toward boundary scan
