I delayed answering this one in the hope that someone out there had a whizzbang
answer. Here is my 1cents worth:
The moulding compound has to wet the surface in order to adhere to it. If the wetting
action is too strong then there will be wicking. Wicking is normally controlled by
working with both the epoxy and lead frame supplier. The epoxy supplier to ensure
that his formulation does not contain ingredients which are known to be mobi;le and
therefore likely to
wick, and the lead frame supplier to control his plating so as to give a surface
which is not "too active" and thus encourage wicking. The major leadframe
suppliers/plating houses for lead frames are familiar with this, for example from
supplying IC houses where epoxies are used for die attach and subsequent transfer
moulding. This might be difficult for you in view of all the processing your
leadframes go through in your plant.
A usual cause of wicking is if the surface is too clean so it might be useful for you
to reivew your cleaning processes as well as talk to your epoxy supplier. Certainly
you need to passivate the leadframe surface in some way. Sometimes just building in a
delay is sufficient.
Sorry not to be more precise/factual, but wicking can be an intractable
problem and hard to solve by much other than empirical methods. [As can be seen from
the other people working on it for 10 years...]

Best of luck

Mike Fenner

BSP, OX15 4JQ, England
T: +44 1295 722 992
M: +44 789 999 7715
F: +44 1295 720 937

-----Original Message-----
From: Eric Lodangco <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: 04 September 1999 20:04
Subject: Re: [TN] Epoxy (clear or black) wicking on leads? How to control and and
possible root causes.


Just would like to correct my item # 7 on the processes each unit will
undergo.Curing is "2 hours" minimumn not 8 hours - sorry..

Any inputs - please read my message below.

>From: Enrique Lodangco <[log in to unmask]>
>Reply-To: "TechNet E-Mail Forum." <[log in to unmask]>,        Enrique
>Lodangco <[log in to unmask]>
>To: [log in to unmask]
>Subject: [TN] Epoxy (clear or black) wicking on leads? How to control and
>            and              possible root causes.
>Date: Sun, 5 Sep 1999 01:49:53 +0800
>MIME-Version: 1.0
>From [log in to unmask] Sat Sep 04 10:49:14 1999
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>
>Hi to all of you!
>
>I'm one of the new members of this forum and for a start I just would like
>to know more about controlling epoxy wicking (capillary effect) on IC
>leads.
>
>We are currently assembling module devices (Non Volatile SRAM's) for one of
>our customers. Our biggest problem now is on how to control this phenomenon
>during encapsulation process. This customer of ours have been doing this
>for almost 10 years but problem was not eliminated. They just recently
>transferred these operations (consigned everything) to us together with the
>said problem.
>
>To give you a background:
>
>1. We manufacture these SRAM's or modules using leadframes (tin plated,
>alloy 42 base metal) as the leads.
>2. Major processes these units undergo - (1) Screen printing, (2)Manual
>loading of DIPs and Lead frames onto the PCB (3) Reflow Soldering (4)
>Aqueous Cleaning (5) Manual soldering of batteries or crystal on PCB (6)
>Encapsulation - using part A and B epoxy (catalyst and resin) mixed and
>dispensed on a tub (using an encapsulation machine) where the unit will be
>inserted  (7) then cured in the oven for a minimum of 8 hours @ 70ºC.
>
>IF we are to consider that there are no handling issues and machine problem
>-
>a. how do we control this phenomenon?
>b. What are the possible root causes of this phenomenon (wicking or
>capillary effect) during encapsulation process?
>c. How do I know if it is a material problem?
>d. Effects of temperature and humidity on these Lead frames?
>
>
>Any inputs will be highly appreciated.. Thanks.
>
>For your attention:
>
>Eric Lodangco
>FASTECH ADVANCED ASSEMBLY
>Philippines
>

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