05-2006, Tema Förbindningsteknik (7 Mbyte, pdf) - Elektroniktidningen

The Print: To start at the beginning, what
needs to be changed to produce RoHS
compliant products?
Vidar Wernöe: First of all designers need
to find components that comply with the
directive. Secondly they need to manufacture
the product in a leadfree process. As for the
banned substances, the issue is about removing
lead without affecting the product`s long term
reliability.
The P: And?
VW: Well, essentially there are several key
problems that arise, at the component, PCB
and layout levels and at a political one. For the
first; when using the viable alternatives such
as a tin/silver/copper compound (SAC), much
higher soldering temperatures are required
(around 40ºC higher) which can lead to serious
overheating problems. Such problems may occur
where products include components of greatly
different thermal mass e.g in power electronics.
Surface mount electrolytic capacitors, for
example, are temperature-sensitive and
overheating can cause irreparable damage.
The P: Can they not be developed to
withstand more heat?
VW: Yes they can. The Japanese, for example,
have been doing leadfree soldering for
some time now, but mainly for consumer
electronics. For industrial products we have a
lack of reliability data and this is a problem. The
production process window has, in leadfree,
become much tighter and this requires a careful
CAD version of heat reliefs
design. Designers need to analyse not only the
electrical data of a component but also examine
the the process temperature specifications.
Ideally a component should sustain a process
temperature of 260 ºC. Components exposed
for over-temperature may cause faults that may
not be detected straightaway in the final tests.
So designers need to go through each single
component used, in order to check its RoHS
compliance, electrical data and the soldering
parameters. This can easily entail checking
the data of more than 1000s of part numbers
for each single new product design or design
change/product update. There is also a key
political issue. Here in Sweden for example
we have asked the Swedish authoritative
body KEMI for guidance regarding proving
compliance and what kind of documentation
they want us to show. They have declined to
give such guidelines however and it may cause
tremendous additional work if it has to be done
retrospectively after a development project is
closed.
The P: You also mentioned PCBs and layout?
VW: Yes. Again because of the effects of the
higher temperatures, the board can warp in
the surface mount production process if there
is an imbalance in the distribution of copper
used. A recommendation is to use “teardrops”
in the layout design. This is where, when a wire
connects to a via, it does not go straight in but
the end is teardrop-formed for extra mass to
diminish the chances of it disconnecting when
heated up. In addition, for larger hole-mounted
components that connect to a larger copper
layer, so-called “heat reliefs” are required to
be able to get an acceptable solder result.
Another recommendation is to widen the hole
slightly. During a leadfree soldering process,
the PCB will be exposed to higher temperatures
than in the past. As a consequence the PCB
matieral chosen is an issue but the key issue is
not to overspecify. Polyclad FR4 370 has better
qualities than standard FR4. It has a high Tg, less
z-value expansion at higher temperatures and
delamination takes longer, but the higher costs
must be weighed up against precise needs.
The P: Inspite of the mammoth amounts of
extra work, are the efforts worth it?
VW: Oh yes. In about a year and a half`s time,
I think the dust will have settled and even
those getting to grips with the issues late will
be producing good quality products with long
reliability guarantees, but there is still some way
to go at a political level to ensure that RoHS
compliancy can be proven and audited in an
appropriate and consistent manner. My advice is
to ask and look for other company`s experience
in RoHS design. We at Elektronikkonsult have,
for example, published our best practices on our
website. This information can be downloaded at
www.elektronikkonsult.se
The P: Finally, we have to ask. As a customer
of Elprint since 2004 are you a happy one?
VW: The Elprint service is reliable, quick, cost-
effective and with a wide product range and
choice of materials so the answer must be yes!
Heat reliefs are recommended for larger
hole-mounted components