Hi Edward,
Circuits Assembly Magazine published an interesting 3-parts series about the
wetting balance, which is a good followup to Bev's excellent explanation:
http://www.circuitsassembly.com/online/0308/0308wetting.shtml
http://circuitsassembly.com/online/0309/0309obrien.shtml
http://circuitsassembly.com/online/0310/0310obrien.shtml
Daan Terstegge
http://www.smtinfo.net
----- Original Message -----
From: "Bev Christian" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, January 22, 2004 5:07 PM
Subject: Re: [TN] basic info on soldering
> Ed,
> Where does one start?!
>
> HMM, the primary solderability test for years has essentially been dip and
look - either manual or supposedly done with mechanical dippers. But the
bottom line is one looked at the soldered sample and determined the amount
of surface that was in contact with the solder and calculated how much was
wetted (covered) by the solder. Now this works well for "giant" dips and
the color coded resistors we played with as kids, BUT hand dipping sure
doesn't work too well for 0402's and 0201's!!! And as George Wenger has
pointed out, for exact dipping nothing works better than a wetting balance!
>
> For leaded devices, chip components and LCC's the way to go today is
really to use a wetting balance. I will use an analogy to explain how it
works. If you stand on a bathroom scale, you weigh so much. If a small
child then comes along and pulls on your pant leg, the scale says you weight
more. It is a dumb scale. It cannot tell the difference between your
weight and a force applied to you. A wetting balance is nothing but a
balance with an internal clock. When the molten solder wets the component,
the surface tension/buoyancy (more later) wets/pushes up on the component
and the balance part of the wetting balance/solderability
tester/meniscograph (spelling?) measures this change with time and display
it with force changes on the y axis and time on the x-axis. In NA and
Europe the machines available usually show positive wetting forces above the
zero line and in Japan it is often the other way around.
>
> The wetting balance curves often have the first part below the zero x
line. This is because the sample has displaced the molten solder and
Archimedes principle is in effect - the weight of the water displaced....
(of course in this case it is the weight of the molten solder). As the
sample heats up and the flux is activated the sample component surface is
wetted by the molten solder and the negative meniscus becomes smaller and
smaller and finally zero. As the meniscus becomes positive and the
component is essentially being pulled down (like the kid pulling on the pant
leg) the wetting balance registers a positive force.
>
> Now sometimes the initially wetting is so fast that you will not see the
dip below the zero x line. The wetting curve will be almost vertical and
then it will usually level off. This is because the wetting force is
fighting gravity. The dense solder is sticking to the component lead above
the equilibrium surface but gravity is trying to pull it back down. The
meniscus will only rise to the point where the wetting force equals gravity.
This is all captured in the basic equation for the wetting balance, but I am
just too lazy to go into that right now.
>
> Now I said the curve usually levels off. There are several cases where it
will not. If the component is a thermal heat sink the curve may have a
stair step shape. If the part has crummy, but not terrible wetting, it
might wet continually but slowly. And then sometimes it will level off and
then slowly decrease. The latter is due to dewetting.
>
> OK, you can look at a curve and an experienced user of the equipment can
usually say by looking at the curve whether the component is good or bad.
But this is a machine that can produce numbers. What exactly can you get?
Well there are a variety - time to reach the buoyancy line, time to two
thirds of the maximum wetting force, maximum force, force at time x, %
dewetting, area under the curve. I am sure that others can give you a
couple more. Suffice to say they are either time or force based.
>
> I am NOT going to go into detail on industry standards here. I will say
that the IPC specs are dated (being worked on) and do not list the wetting
balance as an "official" method. I haven't got my EU specs in front of me
(My office is in another building.) but I can tell you the ones about the
wetting balance are pretty detailed. For the life of me I can't remember if
they list pass/fail criteria. Personally I use T2/3Fmax equal to or less
than 1 second and Force at 2 seconds equal to or greater than 25% of
theoretical max and force at five seconds 90% of force at two seconds (to
allow for some dewetting).
>
> And finally you have to remember that any type of solderability testing is
just a snapshot. It has no statistical validity for a situation where you
are fighting a 200 ppm problem. Don't believe me? Get a stats master to
work out how many samples you would have to test to be 95% confident that
what you are seeing is real!! Of course this is not going to stop me from
using the info to beat a supplier over the head if I can. :)
>
> Hope this helps.
>
> regards,
> Bev Christian
> Research in Motion
>
> -----Original Message-----
> From: Edward Szpruch [mailto:[log in to unmask]]
> Sent: January 22, 2004 9:57 AM
> To: [log in to unmask]
> Subject: [TN] basic info on soldering
>
>
> Hi all,
> I am working on different finishes for "lead free" applications .Recently
I
> got results of solderability tests as graphs showing "wetting force vs
time"
> and "contact wetting angle vs time" , but I do not know how to
interpretate
> it.
> I would appreciate basic info on this stuff.
> thanks
> Edward
>
> Edward Szpruch
> Eltek Ltd
> P.O.Box 159 ; 49101 Petah Tikva Israel
> Tel ++972 3 9395050 , Fax ++972 3 9309581
> e-mail [log in to unmask]
>
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