aemeijers wrote:

On 9/25/2010 12:30 AM, Michael A. Terrell wrote:

Steve wrote:

"The Daring wrote in message
(snip)
even the 13SEER condensers and heat pumps that I install have mother boards
in them, as do the gas furnaces I install. Lots of surface mount stuff,
double layer boards, optical latching relay instead of a contactor, serial
controlled, etc.


I was testing and repairing 16 layer reflowed boards that cost over
$8,000 for the components. If the board house was careless handling the
blanks you could have over 1000 bad solder joints. I spent a lot of
time looking through a stereo microscope, with a hot soldering iron in
one hand.


And nobody sees a basic problem with design philosophy here? Control
circuits for an HVAC system need to be closer to an anvil, than to
avionics, to meet design lifespan if nothing else.


Sigh. It would be great if HVAC electronics were built to Avionics
standards. For one thing, Avionics is 100% exempt from lead free
solder. For another, equipment design is always at the 'Mission
Critical' level. Avionics are built for long life, with little or no
maintenance. It is usually there for the life of the aircraft, unless
that series undergoes an across the board redesign. Of course, it would
double or triple the retail costs of the controller.

One thing I've seen in a lot of failed programmable thermostats is
that they use slide switches that are installed before the boards are
cleaned. Then the board cleaner washes out all of the contact
lubricant.


And they need to be
field-repairable. If it can't be at component level, there needs to be
an industry standard for board interfaces and mounting, so that the unit
cost of boards can be low enough to repair at board-swap level. (Yeah, I
know, there are always plenty of standards to choose from.)


How do you propose to do that? If you make an interface to handle
any possibility, you add a lot of costs. Surface Mount is actually
easier to repair, if you know what you are doing and less susceptible to
board damage if you know how to solder.


If you are a hack at electronics, then all bets are off.


I know nothing about HVAC beyond 'no worky - pick up phone'. But I have
worked in computers at the field support interface for 20-some years,
and have run into similar issues. Engineers in that field violate KISS
principle on a regular basis as well, and component level repair
vanished years ago.


Strange, because I still repair some motherboards. A lot of failures
are bad low ESR electrolytics, or cracked ROHS solder joints. Until I
ended up 100% disabled, I repaired 68340 based embedded controllers at
the Microdyne factory. I also tested and repaired all of the boards in
our dual receiver DSP based RCB-2000 telemetry products.

Contrary to popular belief, having schematics isn't always a help.
OTOH, having the datasheets for the ICs is.

A lot of techs have no idea why some things are done. For instance,
I see people bitch about 'Those morons put the electrolytics too damn
close to the CPU! Why can't the *******s put them on a corner of the
motherboard, away from the heat?" Well, they carry a high ripple
current, so they are self heating. The CPU us a high current, low
voltage device, so the regulator has to be close to the load. Long
traces would radiate a lot of EMI. A lot of power would be wasted
heating the copper traces, and the inductance would screw up the
regulation.

Buy a ESR meter and test the electrolytics on those boards. A little
ripple can cause all kinds of problems. 30 years ago I was doing bulk
repairs on 'Linear' brand garage door opener controls. Every one of
them needed new electrolytics. They were always mounted in hot
locations, and powered 24/7. Similar to the environment in HVAC
systems.

One thing that makes it hard to repair those boards is that the techs
don't use logical troubleshooting methods. They use hunches, or just
shotgun a board till it starts working, or they destroy it with sloppy
soldering or installing parts wrong.


--
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enough left over to pay them.