Bob,
Bellow are some e-mail about that issue sent to Paul Gill at days 11 and 12
of may that will be useful for you ;
Paul,
Multicore are currently developing an anti tombstoning paste. We ran a trial
with it last week
on some dummy boards and parts with excellent results. I do not think that
it is released to the
market yet, but it is definately worth investigating. We placed 4,600
capacitors, and got no
tombstoned part after reflow. The aperture pad ratio was 1:1 using a 5 thou
stencil.
We had run the same boards and parts a few weeks earlier with another solder
paste and got
approximately 50 parts tombstoned out of the 4,600 placed.
Martin Bourke
SCI Ireland
[log in to unmask]
Paul,
You didn't mention if this was a new phenomenon on a new design or if its a
continuous problem, anyway you asked for them, so here are my two cents
worth...
If this is a new condition, you must question have CAD model land geometries
been modified recently?
Does the stencil match land geometries. Has a new stencil supplier been
engaged recently?
Obviously tombstoning is caused from force imbalances during the reflow
soldering process.
It can be caused by:
1) Uneven oven profile/heating causing a differential across the component
terminals.
2) Uneven paste deposition on the lands.
3) Insufficient tack force of the solder paste to hold the component in
place during reflow.
4) Excessive movement during and after the reflow operation can cause
component misalignment which could result in tombstoning.
5) Inadequate placement force to make intimate contact between the paste and
the termination of the pads can also be a cause.
Possible solutions include:
1) Increasing the preheating temperature (following the recommended
guidelines) so that the temperature differential between the terminations is
low at the time of reflow.
2) Ensure consistent solder paste deposit heights between pads via a vision
system designed to measure solder paste deposition height.
3) Avoid inefficient tack force by avoiding extreme environmental
conditions.
4) Minimizing the amount of movement the assembly sees during reflow.
6) Increasing component placement force to ensure contact of the component
terminal to the solder paste deposit (not too much because bridging may
occur if it is too high or if very tight component proximities).
Environmental conditions worth questioning:
What is the temperature of the solderpaste?
Too high or low temperature conditions or too high or low relative humidity
can have an effect.
Too hot and the solder paste may slump which can lead to bridging etc.
Too cold and solder paste may become too thick to yield good print
definition
Are you experiencing stencil clogging?
If the solder paste is exposed to extreme high or low humidity, similar
problems will result.
The recommended temperature the solder paste should be at when printing is
70-77°F, the recommended humidity is between 35-65% RH.
These issues were derived from a handout received from Kester Solder company
a few years ago but I'm sure are still valid!
Good Luck!
Mike Beckman
ATD DPD Substrate Design Integration
[log in to unmask]
Ofc. 602 554-4232
Paul: Try this out:
http://www.plexus.com/In_The_News/RecentPresentations/recentpresentations.h
tml
Andy Mackie
Paul,
Five years ago we conduct a study to better understand tombstone at
0603 components ( we had a big problem that time !!! ), but I moved from
that company and could remember only the highlights, may be with this
information you can apply some experiments to 0402.
The experiment were conducted with prototype boards with 11 different
pad configuration and 3 types of solder paste
Results
Capacitors are more sensitive to tombstone than resistors
Solder Paste with same comercial characteristcs ( Type 3, 63/37 , No
Clean ) gave different results, at that time the best result we get with
Alpha Metals 390 DH4
Pad design with more solder volume applied get more tombstone
results
Pad design with more distance between the two components pads (
center to center ) get more tombstone
Different solder profile were not tested but be carefull with fast
ramp up.
With this experiment we choose the better pad pattern with the
better solder paste to solve our problem. You can easily start
evaluating different solder paste than goes to pad design.
I believe with 0402 you have to be concerned with placement accuracy
also.
Until the changes at our process were not made we had to use glue at
top side process ( we applied solder paste and then glue with a siringe
dispense machine ) to avoid tombstone at 0603 component.
Like americans says " This is my two cents ... "
Jorge Dourado de Santana
Maintenance / Process Engr
Microtec - Brazil
If you are restricting yourself to changing paste, dot size and process
parameters then you should be thinking in terms of speed of wetting. This
will be effected by profile, paste type and atmosphere.
Adjustments should be to slow the rate of wetting down whilst still
achieving
the total activity needed for joint formation. So go for air and a higher
solids type no clean. Adjusting your profile accordingly. [Some
manufacturers
- vertantly or inadvertantly - have "slow pastes" but generally if they do
they will tend to be the higher solid ones]
Mike Fenner
> -----Original Message-----
> From: Furrow, Robert Gordon (Bob) [SMTP:[log in to unmask]]
> Sent: 25 de Maio de 1999 09:29
> To: [log in to unmask]
> Subject: [TN] Tombstoning of 0402's
>
> Good Day Everyone,
>
> We have been running some experiments with different pad sizes / shapes
> for
> reflow of 0402 type components. There are definitely differences in the
> amount of tombstoning seen, but in all cases we experienced an
> unacceptable
> level of defect. We run a BTU VIP98N oven with nitrogen inert and a fairly
> standard ramp / soak / spike type profile. The paste is an eutectic no
> clean
> that has been on the market for several years. Has anyone any suggestions
> in
> reducing the number of tombstones on 0402's? We have run 0603 successfully
> for quite awhile. Feel free to answer me directly (by email or phone) or
> via
> the TechNet if you deem it of general interest.
>
> Thanks,
> Robert Furrow
> SMT Process Engineer
> 978-960-3224
> [log in to unmask]
>
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