Is this due to RoHS solder?

[Jeff Liebermann]

On Fri, 09 Dec 2011 08:31:16 -0600, Jim Yanik <jyanik@abuse.gov>
wrote:

I think the solder crystallizes with age,then thermal cycling cracks it.

Nope. Alloys do not crystallize unless the component metals separate.
That's not going to happen in a fairly short period of time. What
might happen after perhaps 30-50 years is the solder form "grains"
thus separating the tin and lead. Add a little electrolytic action at
the grain boundaries, and we have a crack.

the automotive environment is very tough,hard on electronics.

Yep. The vehicle operating temperatures (-40C to +125C) are similar
to the mil spec electronics temp range (-55C to +125C).

lots of thermal cycling,shock and vibration.

Not thermal cycling. Copper and solder have almost the same coef of
thermal expansion:
solder = 13*10^-6 in/in/F
copper = 10*10^-6 in/in/F
The difference would need to be much more for the solder to
mechanically separate from the copper due to thermal expansion.

However, if the relay is mounted directly onto the PCB, with no
provisions for absorbing motion (shock or vibration), the combination
of PCB flex and mechanical stresses are going to break the joint. I've
seen PCB's where components will "retract" slightly when I hit the
connection with a soldering iron. That means the joint was
pre-tensioned or under stress from the original soldering and just
waiting to break. That's why I suggested a rubber washer or preform
under the big components.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS

 

[N_Cook]

Jeff Liebermann <jeffl@cruzio.com> wrote in message
news:738de79p8jh85525qed9l0jdosr5p8rp1m@4ax.com...

On Thu, 08 Dec 2011 18:37:06 -0500, Boris Mohar
borism_void_@sympatico.ca> wrote:

Took apart a Volvo 850 fuel pump relay. Cracks everywhere.
http://www.viatrack.ca/Misc/FP%20Relay/

What year Volvo? The Volvo 850 was made from 1992 to 1997. This was
somewhat before RoHS solder was introduced en masse. Looking at the
photos and guessing by the age, methinks it's just a typical lead-tin
cracked solder joint produced by excessive vibration. It's not that
unusual for fuel pumps:
http://www.volvoclub.org.uk/faq/EngineFuelinjection.html
Scroll down to "Testing or Repairing Bad Fuel Injection Relay".
Also:
http://www.tegger.com/hondafaq/mainrelayoperation/badmainrelay.html
and others. Note the relay boards which look much like your photos.

In a past life, I used to design marine radios. We had solder
cracking problems on unsupported heavy parts (xformers, power
resistors, big electrolytics, etc) due to engine vibration and shock.
Oddly, the solution was NOT to support the part, because the load was
too great and would crack the glue. It was to decrease the size of
the PCB hole down to a tight fit. That would cause the vibrational
loads to be transferred to the PCB and therefore reduce the stresses
on the solder joint. With a large hole and fairly soft solder, the
solder tends to get extruded out of the hole eventually forming a
annual ring around the lead. (Hint: copper leads are harder and
stiffer than solder).

Somewhat later, we started using rubber washers and preforms under the
heavy components to help absorb the load. Imported electronics often
uses yellow acrylic goo to support top heavy parts, but makes rework
and quality control a PITA.

If you want to see how such a joint is formed, shove a heavy copper
component pin into a loose PCB hole and solder the pad. Find a
vibratory scraper or ultrasonic scaler that will accept a chuck. If
you have time, a vibrating toothbrush will do it after a few hours.
Connect the chuck or clamp to the wire lead. Vibrate until the lead
comes loose. You'll find that the crack somewhat resembles the
annular ring crack.


--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS

PCB holes somewhat larger than the component pins always, given time, would
cause ring cracks in solder but with PbF occurs in just a year or two. For
things like pcb mounted multi-terminal transformers they could have an extra
machining stage. A conical grinding or milling stage, per hole, on the side
that accepts the component so that larger holes could be avoided and still
not mess-up the hand alignment/placement due to microscopic , but in spec,
misalignment of the pins

 

[Boris Mohar]

On Mon, 12 Dec 2011 18:04:35 -0800, Jeff Liebermann <jeffl@cruzio.com> wrote:

On Fri, 09 Dec 2011 08:31:16 -0600, Jim Yanik <jyanik@abuse.gov
wrote:

I think the solder crystallizes with age,then thermal cycling cracks it.

Nope. Alloys do not crystallize unless the component metals separate.
That's not going to happen in a fairly short period of time. What
might happen after perhaps 30-50 years is the solder form "grains"
thus separating the tin and lead. Add a little electrolytic action at
the grain boundaries, and we have a crack.

the automotive environment is very tough,hard on electronics.

Yep. The vehicle operating temperatures (-40C to +125C) are similar
to the mil spec electronics temp range (-55C to +125C).

lots of thermal cycling,shock and vibration.

Not thermal cycling. Copper and solder have almost the same coef of
thermal expansion:
solder = 13*10^-6 in/in/F
copper = 10*10^-6 in/in/F
The difference would need to be much more for the solder to
mechanically separate from the copper due to thermal expansion.

However, if the relay is mounted directly onto the PCB, with no
provisions for absorbing motion (shock or vibration), the combination
of PCB flex and mechanical stresses are going to break the joint. I've
seen PCB's where components will "retract" slightly when I hit the
connection with a soldering iron. That means the joint was
pre-tensioned or under stress from the original soldering and just
waiting to break. That's why I suggested a rubber washer or preform
under the big components.

Relay armature, diodes and resistors on this board are all clad Iron.

--
Boris

 

[Jeff Liebermann]

On Tue, 13 Dec 2011 08:34:06 -0000, "N_Cook" <diverse@tcp.co.uk>
wrote:

PCB holes somewhat larger than the component pins always, given time, would
cause ring cracks in solder but with PbF occurs in just a year or two.

PbF is Lead Fluoride in chemical terms. Please find a better abrev.

For
things like pcb mounted multi-terminal transformers they could have an extra
machining stage. A conical grinding or milling stage, per hole, on the side
that accepts the component so that larger holes could be avoided and still
not mess-up the hand alignment/placement due to microscopic , but in spec,
misalignment of the pins

It's been a while since I've ordered production PCB's, but as I
recall, the number of drill changes has a huge impact on the cost.
Auto manufacturers are VERY sensitive to costs. As a result, it's
more common to use cylindrical holes and wedge shapped component pins
on things like xformers and relays. These have the added bonus of
being self supporting during wave soldering.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS

 

[Mike Tomlinson]

In article <2ei2e75p1a6q0j3lkdb68h7ija4cfe7s5g@4ax.com>, Boris Mohar
<borism_void_@sympatico.ca> writes

Took apart a Volvo 850 fuel pump relay. Cracks everywhere.

Date of manufacture 1996, so it's 16 years old. What do you expect?

--
(\__/)
(='.'=)
(")_(")

 

[Ian Field]

"Mike Tomlinson" <mike@jasper.org.uk> wrote in message
news:laCBh6Crbe6OFwXS@jasper.org.uk...

In article <2ei2e75p1a6q0j3lkdb68h7ija4cfe7s5g@4ax.com>, Boris Mohar
borism_void_@sympatico.ca> writes

Took apart a Volvo 850 fuel pump relay. Cracks everywhere.

Date of manufacture 1996, so it's 16 years old. What do you expect?

Probably more than a 50yr old bakelite radio that the soldering was done
with a hearth poker style iron heated on a gas burner and rubbed on a block
of sal-ammoniac before tinning because the solder had no flux of its own.