Re: Soldering SMT Components
- From: Glen Walpert <nospam@xxxxxxxxx>
- Date: Wed, 23 Apr 2008 14:24:32 GMT
On Wed, 23 Apr 2008 05:15:20 GMT, JosephKK <quiettechblue@xxxxxxxxx>
wrote:
On Tue, 15 Apr 2008 00:50:58 GMT, Glen Walpert <nospam@xxxxxxxxx>
wrote:
On Mon, 14 Apr 2008 11:58:59 -0700, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Mon, 14 Apr 2008 11:49:27 -0700 (PDT), jd_lark@xxxxxxxxx wrote:
I'm looking for Do-it Yourselft articles on soldering surface mount
components. I can't afford the IR equipment that commercial board
houses use. I'm particularly interested in soldering the new ROHS
components that require lead free solder. The lead free solders
require higher temperatures. Also, do board coatings prevent problems
with tin whiskers that grow from the lead free solders?
Tin whiskers do not grow from lead-free solders, where did you get
that crazy idea? Tin whiskers grow from tin plating and only tin
plating.
It's easy to solder most surface-mount parts by hand; just practise a
bit.
And it's OK to use rosin-flux leaded (63/37) solder, even to solder
lead-free parts. Better in fact.
As far as I know, nothing prevents whiskers, not even coatings. Except
lead, of course.
Lead does not prevent tin whiskers any better than any other means of
pinning dislocations in tin, such as any of the alloys used as
lead-free solders. No one has ever been able to show me believable
evidence that so much as a single tin whisker has ever grown from any
currently used lead-free solder alloy, and I doubt they ever will, it
is simply not a valid reliability issue.
Much of it has been shown in references here. Maybe you should look
some more.
I have read every reference posted here that I noticed, along with at
least 600 other articles on the subject over the last 10 years, much
of it including actual test results with complete description of
materials, processing parameters, test conditions and results with
statistical analysis. Nothing I have ever read regarding tin whiskers
from lead-free solder holds up to the slightest bit of scrutiny.
Asian manufacturers have been cranking out millions of lead free
assemblies per month for years now. Where are the failures?
Statistical analysis shows improved yield and reliability, and
absolutely ZERO failures have been reported due to tin whiskers
growing from lead-free solder. Compare to the failure numbers from
HDD rotating elements, fan failure, electrolytic capacitors, etc., the
whole lead-free reliability scare is clearly much ado about nothing.
Complaining about problems with lead-free these days makes as much
sense as still worrying about the Y2K bug. Forget about it, the
conversion is over and done with for all but the smallest
manufacturers, and reliability of the assembled circuits has continued
to increase.
Lead is in fact not a very good means of preventing tin whiskers, due
to well know problem of poor fatigue strength of tin-lead solders.
All high-rel applications which need to withstand shock and vibration
will be converting to lead-free; not the lowest cost SAC alloy but
high performance lead-free alloys which are now being qualified (a
lengthy process) but which clearly blow the socks off tin-lead in
strength, ductility, shock resistance and fatigue life; all of the
properties important for reliability except processing temperature are
greatly improved and the processing temp for these alloys is only
slightly higher than tin-lead. See for instance:
<http://smt.pennnet.com/display_article/323445/35/ARTCL/none/none/1/STEP-3:-Holistic-Lead-free-Reliability/>
The down sides to lead-free are slightly higher cost and far higher
complexity of specifying the optimum alloy, flux, and processing
parameters, plus a need for tighter process controls.
This is not a reasonable forum for learning about soldering or PCB
assembly in general, the issue is too complex and almost no one here
is interested in it enough to have read up on the subject. Read the
trade rags like SMT <http://smt.pennnet.com/home.cfm> and Printed
Circuit Design and Fab <http://www.pcdandf.com/cms/> for a decent
introduction to the subject.
There is way too much serious studies and reports that contradict you.
Name one. Post a link, and I will explain what is wrong with it or
why it does not apply to current materials and processes.
No one who is serious about high reliability is failing to investigate
the advantages of lead-free. It is now clear that tin-lead has no
future in high-rel PCB assembly; its fatigue and shock resistance
properties are not even in the ball park of the latest high
performance lead-free alloys. Read the literature.
.
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