Mohit,

Many of the variables depend on the type of machine, as well as the component
you are placing.  When we calibrate equipment, we like to place on double
sided tape to eliminate misplacement due to any small movements of the component
once the component hits the paste.  If placing off of a rotary turret machine,
the movement in the paste could be extensive.  Next insure that the nozzle being
used for the particular component is of adequate size to allow sufficient vacuum
presure(assuming vision alignment rather than mechanical alignment).  The 
component may slip on the nozzle after the vision alignment step- along the
same lines make sure all rotations to the components are being made prior to
vision alignment.

Depending on the vision system you are using(binary or grey scaling), and the
type of lighting(front or back) you might run into some problems.  
Many parts have plastic flashing, which may result in component to component 
varience when using backlighting (looking at a silhouette).  Binary vision 
will also give you some small varience depending on pixel size.

If using a leaded component front lighting should be used to place the component
by viewing the component leads, backlighting is sufficient for leadless 
passives.

The best bet is to invest in some dummy components, we use glass cutouts with
a component outlined on the glass, and place these on a calibrated glass board
so they can easily be measured on an optical comparitor to +/-.00001".
A typical IC placement machine should place at +/-.002" with a CpK over 1.00
(6 sigma), while a typical chipshooter is in the +/-.004"-.006".

If placement is poor enough to identify by first glance, check for component
slippage on the nozzle, and/or get the equipment rep in there!

                                                              Steve A

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