Bev has a good point- where the part is positioned after reflow is a function of solder wetting and surface tension of the solder after reflowed- in addition to Bev's comments I add a few:
copper distribution - where one part lead heats up more than another lead (tombstoning on chip parts)
surface finish solderability - which is also related to board (ENIG, OSP or HASL) or part finish (tin/lead, matte tin, tin-palladium)
solder paste viscosity changes during reflow (as thixotropic modifiers and fluxes heat up they may slump causing variations in how much solder may wet back into the solder joint).
From experience I would expect more changes in position in Ball Grid Arrays (and derivatives of them) and smaller chip components (1210 or smaller components).
>>> [log in to unmask] 02/16/04 04:02PM >>>
Jeff,
Short answer - not that I know of. But our designers sure wish there was too. They want the plant to actually guarantee the vertical distance off the board for one part!!!!
Why can't they?
Slight variations in flux/solder sphere ratio in each dollop of solder paste
Relatively wide range of how much solder is put down through the stencil,
Very slight variations in pad size and orientation of stencil to board,
Minor variations in air currents, temperature
Variations in solderability lead to lead
Variations in lead shape lead to lead
And I am sure that other TechNetters will point out other variables.
Good luck, you're going to need it.
regards,
Bev Christian
Research in Motion
-----Original Message-----
From: McGlaughlin, Jeffrey A [mailto:[log in to unmask]]
Sent: February 16, 2004 3:54 PM
To: [log in to unmask]
Subject: [TN] Pick-and-place Tolerance
Help all -
I need to know if there is a predictable tolerance on parts placement
AFTER REFLOW. I am fairly sure the pick-and-place equipment we" use is
stated to be repeatable to 0.005" with an angular accuracy of 1/2
degree, but that is before reflow is there a metric or formula for
determining true position tolerance of component placement for post
reflow? The reason I am asking is that our mechanical design group has
designer a bezel for a display panel that has a true positional
placement requirement of 0.006 on our finished array of 246 large SMT
LEDs.
Thanks for the help,
Jeffrey A McGlaughlin, CID
Engineering Designer
Battelle Memorial Institute
Columbus Ohio
http://www.battelle.org
[log in to unmask]
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