Mel-
Thanks for the info!
Yes, I can certainly see that the meniscus would prohibit wetting. And,
especially,
in the worst of all situations - meniscus in the solder joint when in a
single sided
board application - the premature failure over time and temperature extremes
seems quite likely.
What I'm also concerned about, though, - and this is the predominate debate
point, here - involves potentially compromised PTH's, when the meniscus
in the holes is allowed. Won't the partial blockage of the hole create a
potential
entrapment of flux gases or residues? And, if that is so, it seems like at
least
two problems may result: 1 ) a diminished area of wetting inside the PTH
(and
therefore a weaker mechanical junction of less current carrying capability)
and
2) the potential for a breakdown of that hole plating due to either
corrosion or
expansion and contraction of the trapped materials (or would it be due to
the
relative differences in TCEs of the plated hole and the trapped materials?).
A third possiblity might include the breakdown due to increased heat
generated
at these highly resistive pockets.
I know it sounds like I'm answering my own question, but I haven't seen any
reports or articles that back me up. I'm questioning my own paranoia, I
guess.
Y'see, if those points are valid, then the justification for the spacing is
self-evident.
If not, well....Good! I can relax a little.
So, am I out to lunch on this or what?
Lunch? Oops, gotta go.
Thanks, again!
----------
From: MTTC
To: internet!ipc.org!TechNet-d; JMcgee
Subject: Re: MeniscusBlockage
Date: Wednesday, November 13, 1996 9:48AM
John,
It is true that the requirements have changed from J-STD-OO1A to B for
meniscus in the solder connection. IPC-A-610B was compatible with the
J-STD-001A.
The condition was downgraded to "should" which is a recommendation (para
1-4, 001B).
We should also consider that there still remain some wetting requirements
for the solder connection in the B revision. Since solder cannot adhere to
meniscus, the wetting characteristics of the connection are affected.
The failure mechanism reflects the fact that solder cannot wet to component
meniscus material. Severe environments could cause the resulting solder
joint to fail. Another consideration was damage to the component, which I
cannot see except in extreme cases.
Personally, I have seen many solder connection failures resulting from this
condition during service life of products (including class 1 and 2). The
condition becomes doubly critical when single sided boards are employed due
to minimal solder strength and intrusion of the meniscus through the PWB.
I don't know of any test reports regarding the condition. If they were
available, it would have been from the Mil Spec world to justify the
Military Standard content and since they traditionally required
consideration or corrective action it became a mute point through years.
Best regards
Mel Parrish
President, MTTC Inc.
On Tue, 12 Nov 1996 10:54:00 +0000 John McGee wrote:
"Looking for clarifications, please...In J-STD-001A, Section 6.6.4.5
-'Meniscus Spacing &Trimming' it was stated
that '...Components SHALL be mounted to provide a visible clearance between
the coating meniscus (on each lead) and the solder connection', making it
mandatory for Class 3; If products in Classes 1 & 2 did not meet the
process
control requirements, this condition was mandatory, too. Otherwise, it was
a Recommended condition for Class 1 or 2.
In my pre-release copy of J-STD-001B, the statement reads ' ... Components
SHOULD be mounted...' in that way. I see no interpretation of the word
'should' in section 1.4 and , as well, no differentiation among the
Classes.
1) Is the interpretation cleared up in the released version of -001B?
2) Has the requirement really changed since the previous revision?
In IPC-A-610B, Section 3.2.4- 'Component Installation-Vertical - Radial
Leaded' and 4.2.3- 'Soldering-Meniscus in Solder', the 'clearance-required'
condition is illustrated. It's clearly shown that meniscus in the PTH is
unacceptable
for Class 3 assemblies. With certain provisos, however, meniscus in the
holes
is acceptable for Classes 1 & 2.
As a Class 2 house (for now), wary of unnecessary costs- such as those
associated with spacers or special lead forming- we periodically debate the
intent of this requirement.
3)The clearance requirement is for more than just inspectability, isn't it?
4) What aspect of reliability is potentially compromised when the meniscus
isallowed in the holes?
5) And in what ways will this breakdown manifest itself?
6) Where may I find technical papers or test data that will help to
illustrate this condition?"
We can be reached at: e-mail: [log in to unmask]
Manufacturing Technology Training Center (MTTC), Inc
543 Graaf Street Phone 619 446-5571
Ridgecrest CA 93555 Fax 619 446-4337
Home Page: http://www1.ridgecrest.ca.us/~mttc/mttc.htm
Bulletin Board: 619 446-4087
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