If you see something disturbing, take off your glasses, and alás, the ugly
thing isn't there any more. The question about magnification is discussed
daily and everywhere. I understand your perplexity.
My view: The important question should not be what magnification you use.
Instead you should ask yourself whether this amount of solder splash is bad
or not for the reliability. To forbid ANY solder on the gold fingers would
be a exaggeration. Next question ought to be why solder is forbidden at all?
It seems as we need take a further step backwards. Why is there gold on the
fingers? Why not tinned fingers? I've seen and examined thousands of boards
the last three decades. Both gold plated and tinned. In general, they are
compatible and equal. In very harsh environment, gold plated fingers win
because gold is 'inert' . Another reason for using gold is the lower and
more reliable insertion resistance if you plug in /out frequently. If you
don't do that, tin may work good enough because of the hermetic micro
connection you get.
So, tin is not a enemy but a possible member. Like gold, also tin works as a
'lubricant' when the connector forks slide over the fingers. Also like gold,
tin creates hermetic micro connections, which is a condition for longlasting
low ohmic resistance. If small amounts of tin exist on the fingers, there
will be no problems. With some exceptions. Namely a) the tin may include
fluxes b) the tin may not be flattened under the sliding contacts but pushed
aside, loosen and become a dangerous debris that can cause electrical
shorts. This was a short view upon the basics of tin contaminating gold.
Will I come to any conclusion? No. It's YOU that must come to a decision,
possibly with the above in mind. To refuse a batch of extremly expensive MIL
boards because of a nearly invisible amount of tin would be madness, as well
as accepting hundreds of solder paste spheres that will jump around. As
always, Dough's 'it depends' is a good start before give instructions to
the operators. By the way, there is a further reason for not follow the
standards thoughtlessly : optical dispersion between individuals. There are
guys who have hawkeyes and those with weak sight. The former notice lots
even without a microscope, while the later does not observe anomalies even
in a microscope.
I can see your impatience (' Gosh! He is talking and talking and never comes
to the point...) so I round up with recommending that you read a few pages I
send offline, from Puligandla Viswanadham's book " Failure Modes and
Mechanisms in Electronic Packages. He has a table with factors that have
impact on connectionsand contacts in general, and tin spots on gold is not
even mentioned ( not saying that his book covers everything ). Standards are
like medicines, we need them but they should be used correctly.
I can hear your relief ( 'My goodness, he is ready...)
Good Luck
Inge
On 20 April 2011 22:13, Kelly Morris <[log in to unmask]> wrote:
> I am looking for some discussion on acceptability of solder on gold fingers
> per IPC-A-610.
> I've searched the archives and have not found my answers there.
>
> IPC-A-610, section 10.1, page 10-3 says:
> Solder, any metal other than gold, or any other contamination in the
> critical contact area of the fingers is a Defect - Class 1,2,3.
>
> At face value, I would interpret that to say that ANY solder found in a
> visual inspection on a gold finger would be considered a defect.
>
> The solder we are seeing is from indiviual solder spheres from the solder
> paste and are being missed by the assemblers and process inspectors. They
> currently believe that if the solder contaminate on the gold finger is not
> visible with a 4x magnifcation, it is not a defect.
>
> The gold fingers under inspection are designed at .040" wide.
> There is one QFP on the PCA that has solder pads that are .011" wide
>
> My question is.....To inspect the gold fingers and determine if a PCA is
> defective or not.....What magnification level is appropriate when inspecting
> these gold fingers.
>
> Information/Discussion:
> Per table 1-2 on page 1-6, suggests for lands > 0.0394 a Magnification for
> inspection of 1.5x to 3x with a max referee of 4x.
>
> However, the last sentence of paragraph 1.8 on page 1-6 states "For
> assemblies with mixed land widths, the greater magnification may be used for
> the entire assembly.
>
> If this PCA has some land widths designed at .011" wide, then according to
> the last sentence of paragraph 1.8, I would say that per table 1-2, an
> inspection magnification of 7.5x to 10x with a max referee magnificaton of
> 20x would be appropriate, and any solder that is visible on a gold finger at
> that magnifcation level is a defect.
>
> Is my interpretation flawed? too stringent? Comments?
>
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