Bridging with solder paste may be associated with this sliding mechanism,
as you say, but that can easily be verified by seeing if the components
have shifted around before reflow. Also, is the bridging just along two
edges of the fine-pitch package, or is it the same on all four sides? This
will be a key thing to note: two sides implies that the conveyor may be to
blame, as long as the bridged sides are in the same plane as the bridged
edges. Was this loader/conveyor system in place beforehand, or is that a
new variable, as well as the oven?

One thing that you may want to consider is the effect of slump of the
solder paste, either cold slump (between printing and reflow) or hot slump
(during reflow). Cold slump may be due to downward-variations in paste
viscosity, or increased placement pressure: or (if it's a water-wash paste)
absorption of moisture.

The key with reducing hot-slump when moving from IR to forced-convection is
to balance the improved ramp rates you get against the loss of solvent due
to the forced convection ("laundry drying in the wind") effect. If the
paste heats up too rapidly with a slow flow of air/gas over it, there may
be a lot of solvent left in the flux that can contribute to slumping.

Hope this helps,

Andy Mackie, PhD (Chairman - IPC Solder Paste Task Group)
Praxair Inc.
777 Old Saw Mill River Road
Tarrytown
NY 10591
Ph: 914-345-6402
Fx: 914-345-6405





Energy Technology Systems <[log in to unmask]> on 02/03/99 01:58:01 PM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
      to Energy Technology Systems <[log in to unmask]>

To:   [log in to unmask]
cc:    (bcc: Andy Mackie)
Subject:  [TN] Solder Bridging




I have a customer who recently replaced a very old IR reflow oven with a
brand new convection one. Since the new oven has been installed they are
noticing an increase in solder bridging on some very fine pitch components.
Even the customer says that they do not suspect the reflow oven but they
cannot place a finger on it. Being the manufacturer of this oven I want to
help prove that it is not the problem as none of our other customers have
ever complained about solder bridging and from my experiences solder
bridging has been reduced dramatically when switching from IR to
Convection.
Can anyone suggest additional places to look.

The area I am most suspect of is the conveyor just prior to reflow. The
reflow has an edge conveyor and the linking conveyor just prior to reflow
is
"power loading" the boards. By power loading I mean the upline conveyor is
running considerably faster so that the boards slide a ways onto the
conveyor prior to slowing to match the reflow conveyor speed. I have heard
that boards can crash into the conveyor pins, other boards, etc. when power
loaded and cause components to shift. Could this be the case?

I know it can not be related to the energy transfer as the delta on their
boards was reduced from 40 degrees C to less than 15 (actually 8 - 15
degrees depending on the board) by switching from IR to convection.

Comments?

Brian

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