Date: Tue, 1 Mar 2011 11:32:58 -0500
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Ghastly examples
In-Reply-To: <061601cbd7c0$b970c820$2c525860$@net>
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At 10:28 PM 2/28/2011, Tom Hargrave wrote:
>All of these new failures are de-wetting type manufacturing defects
>and are nothing like the stress fractures in the first set.
Tom - on the first four that seems a plausible explanation, and
they're the four leads of two components: two large Wima ?polyester?
caps, 1.5uF/100v and 0.15uF/630V, in the ignition drive circuit. The
one with the big arc is the high-voltage one. However, though you
can't see it in the photos, the lead tinning is excellent and uniform
(it wouldn't surprise me if Wima leads were silver-plated), and above
the failure point you can clearly see the sharply acute angle of a
well-wetted joint; so I have considerable doubts.* Incidentally this
is a Bosch 022 board, and contrary to my previous rants the initial
solder is perfectly good - apparently somebody told the production
people to quit messing about and stop using plumbing solder in the
wave machine. It certainly makes forensics easier.
On the rest it's not possible - that raised area is spongy, and if
you rotate the lead gently with pliers you can see the mass rotate
and shift within itself. It's still firmly attached to the lead. A
side note - that's one of the 20-pin ribbons connecting the two
boards; the longer one in fact which is bent in an S-curve; and as
you look along the line you can see the size of the fractured area
and degree of vertical displacement increase smoothly from one end to
the other. The photos are from at or near the worse end.
*Zoltan - for your purposes it doesn't matter much except in that it
opens another book on tinning and lead cleaning and what a properly
wetted joint - which you must achieve in your repairs - looks
like. Short answer on the last: the solder has to join the material
being soldered at a narrow acute angle. If it beads up to the
slightest degree then it's not properly wetting the material and you
have a bad joint. The material is not clean enough or your flux is
defective/not strong enough. Note that using plumbing flux for
electronic soldering is an absolute no-no; it is guaranteed to cause
trouble down the road.
If you're serious* about this, I'll lead you through it (but I won't
and can't do all the work for you - google is your friend), and now
is the time to start learning to solder. Start by watching this
video http://www.youtube.com/watch?v=I_NU2ruzyc4 . It packs an
enormous amount into seven minutes and will give you a lot better
start than most people get. Frankly it's the best training video
I've ever seen on any subject, and I suspect it's not by chance that
the (rapid) speed of his speech is precisely what has been
demonstrated to achieve maximum retention for most people.
*to me that means willing to put a hundred hours and (if you include
a microscope and industrial stand) up to a thousand bucks into
it. Half that if you're really lucky on the scope. Say a couple
hundred or less if you just want to go for the obvious ones and be
able to fix them reliably without hurting the board. People may jump
up at this point and say that's too much money and too many hours and
I say more power to them, they're probably right. But if you're
going to be involved in fixing other people's ECUs, that's the
minimum commitment I require from you before I'll actively help. If
it takes less, great.
Get a practice board just like he used and a grab-bag or two of parts
from Radio shack. They will also sell you a small amount of 60/40
electronic solder (not the hair-thin silver-bearing stuff for surface
mount components), a cheap iron and a stand and sponge, and a little
tin of tip cleaner. Scrub half the board shiny-bright with
Scotch-Brite. If there's a TV repair shop in town, ask if you can
buy a handful of old components that have been lying around for years
and gotten well tarnished. Now take all the above and practice until
you can make a perfect joint every time. That's the first step.
Preview of coming attractions: Removing soldered
components. Repairing damaged traces. Installing and removing
integrated circuits (DIPS). Equipment/materials for above.
At that point you'll have mastered the easiest type of electronic
soldering, the single-sided board; and you'll be ready to begin
reworking double-sided boards which is a lot harder. Principles are
the same but the practice is a good deal more demanding and you'll
need a better iron. As a first step, start looking for some
double-sided boards to practice on. Open up a 2.1l ECU, take it to
your TV shop and explain you need something like that, and
why. Chances are there'll be something lying around or in the
dumpster. Early '80s era computer gear would be ideal as well.
A refinement on visibility of the joints in my pictures: The burned
ones jump out at you. The other two are easily visible in a good
light but you do have to look. The ones I called out as visible
under a 10x loupe are in fact easily seen from the side under a
raking light with a 4x loupe, as the reflectivity is different and
the raised edge of the defect catches the light.
Microscope stuff later.
Yrs,
David
