Michael,

Here are some answers IMHO to your questions.  These are sensitive
topics for some people but here goes:


Michael Yarrow wrote:
>
> Dear TechNetters
>
> I have a question about PCBs using leaded components versus PCBs using SMD
> components...snip...
> (My application involves development of products for a commercial
> environment.  However, it is a requirement that we achieve long MTBFs over
> the standard commercial temperature range).
>
> 1. What are the criteria that should be used when deciding at the beginning
> of a product development cycle whether to use SMD components or leaded
> components on PCBs ?

I believe in a life cycle approach to determining fatigue life
(reliability) of electronic assemblies.  The clock starts after the
board has been assembled (soldered one or both sides).  Any rework or
testing that occurs will start to accumulate damage and reduce life.
Some testing (e.g. Environmental Stress Screening (ESS)) is designed
to induce enough damage to weed out latent defects but not a
significant amount to drastically reduce the life of good parts.  One
rule I have been taught is that ESS damage should be 1% of the total
life cycle.

>
> 2. I know, for example, one such criterion is mechanical stress and
> acceleration of the completed board and assembly.  Above what value of g's
> is SMD-based PWAs yield unacceptable short- and long-term reliability ?
> (This is why, I believe, that the aviation industry continues to use leaded
> component assembly methods).

Most of the avionic industry is going away from through hole parts for
a number of reasons: part obsolescence, increased I/O count & density,
smaller size and weight to name a few.  Engineers have found ways to
make surface mount (SMT) components (leaded, leadless, BGA, COB and
chip scale) work for extreme environments.  One common method is to
mount 2 single sided SMT assemblies to a common heatsink core (6"x6" ~
SEM-E size).  This assembly can be made very rigid (with natural
frequencies above 500 Hz) and can have low thermal resistances.  The
advanced composites heatsink materials such as Aluminum Silicon
Carbide (AlSiC), Beryllium Beryllium Oxide (BeBeO) have high
stiffness, low CTE, and good thermal conductivity.

My point here is that with enough time, money and engineering
resources, SMT can and is used very reliably for avionics.  In fact,
some of the latest designs would not be possible without SMT!

Your questions about a specific g level depend on the type of dynamic
event.  I have seen random vibration inputs for aircraft avionic boxes
over 25 grms and pyroshock (SRS) inputs for spacecraft boxes over 3000
g.  In both cases the boxes used SMT assemblies.  I am sure others out
there have seen higher levels.

The main criteria to consider is to minimize strains and stress
(thermal and dynamic) in the component connections and the PWBs
themselves.  David Steinberg's two books "Cooling Techniques for
Electronic Equipment" and "Vibration Analysis for Electronic
Equipment" are two of the best (but sometimes very conservative) books
in this area.

>
> 3. Any other important criteria that must be considered.
>

One very important step in doing any analysis is to have a good
understanding of the environments that your products will see.  Think
life cycle: everything from in house testing through transportation to
market and finally in service conditions should be considered.

> Thanks in advance
>
> Michael Yarrow
> Techniplan International Pty Ltd
> [log in to unmask]
--
Karl Sweitzer                     voice: 716.47.77546
Eastman Kodak Company             pager: 716.25.33681
800 Lee Road                        fax: 716.47.77293
Rochester, NY 14650-3118         mailto:[log in to unmask]

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