Dear sir,
I would like to tell you I am not the expert, I have ever experienced on
the solder bridging issue.
I don't know the exact information of your process, mat'l type, M/C type,
environment control. I might explain you the potential root cause and
problem solving of each cause. O.K. let 's start.
1. Environment control : If your solder paste is water soluble tpye, it
sensitive to the temperature and humidity the environment temperature and
relative humidity is importance. The high temperature and high humidity can
reduce the solder paste viscosity. Temp should be 21-27 degree C and %RH
should be 30-60%, please try to control at the central point.
2. Solder paste stirring : The solder paste stirring before use should be
gently stirred 2-3 minute and 20-25 rpm. The longer and faster stirring can
break the Thickener of the solder paste and cause to lower paste viscosity,
Thickener is the polymer to hold the solder particle joint together.
3. Screen printing : Please make sure your M/C is not set too high speed ,
should be 25-35 mm/sec and is not too high squeegee pressure, should be 1-3
lb/inch, if you use the squeegee 300 mm the total pressure should be 5-6 Kg.
4. Solder paste height : Too high solder paste has more chance to slump
and short after printing and component mounting, cause to bridged solder.
For SOIC 20 mils, 6-7 mils paste height is O.K.
Please kindly take a look the paste shape after printing, if you find
the slumping paste please let to check all above items.
5. Fixturing : If your process have the fixture and it is the high mass
fixture like a CDM or AL please make sure it cool enough before loading the
unit onto it. It can make paste slumping and short to adjacent pad.
6. Component mounting : Too much placement force can cause the paste
spread out and short the adjacent pad, verify the placement force.
Please kindly check the solder bridging after IC placement again if
the solder bridging is found the items 5 and 6 are required to see.
7. Reflow soldering : Difference heat transferring between PCB pad and IC
lead can cause the problem. Verify reflow profile with two TC wires, IC
solder joint and on the board then take a look the difference, the preheat
temp is required to see. Another, you can try to set the bottom zone higher
than the top zone if you see the difference.
8. Furnace conveyor : Un smooth conveyor moving is one cause to
misalignment component or bridging. Please make sure the conveyor
tightness, too loose conveyor can cause un smooth conveyor moving.
I would like to say for solder bridging problem the reflow process is
the last priority to verify.
These are only a little help from me that I know, hope it can help you.
With best regards,
Noppadol S.
At 09:07 25/7/98 PDT, you wrote:
>Good Morning!
>I hope all of you had a good weekend and are back for an exciting week.
>I had a strange problem of solder short a few days ago. The shorts were
>between 2 pins of an SOIC towards the plastic body, and not at the board
>level. Can the soldering experts of TechNet help me understand this
>strange phenomenon ?
>
>Regards
>Krishna
>
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