how solder pad size affects the component psoitioning?

[limct]

Hi,
I have a question about how a solder pad size affects the positioning of a component after the reflow soldering process.
Say if I make the pad size bigger by 0.1mm all around, does this mean the component is given the possible displacement of 0.1mm about it's intrinsic center point?
If I wanna make the component stays inline with the solder pad in x-direction, can I make the pad size smaller than the component lead in x-direction? Thanks.

Best regards,
ct

 

[itsmoked]

It all comes down to the solder surface tension and menicus. The forces developed are a function of the size of the pins, the number of pins, the type of solder, the pad/pin dimensions, and the weight of the package. So I can't say "oh yeah make the pad x by y and thus shall happen. But in general up to a point the pins center on the pads. The 'point' being after you are out of the meniscus area, more pad in some dimension doesn't change anything.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Noway2]

IPC has specifications governing aspects of how much a component lead is allowed to overhang the pad. While you may be able to make the pad size smaller than the component lead, it is probably not a good idea.

In most cases, on a properly designed footprint, the component will tend to center itself in such a way that it is acceptible. There is often times a lot of 'slop' that is allowable in the pattern footprint to still obtain adequate positioning. The positioning, though, is not usually a critically defined item and you shouldn't have to worry about designing for exact alignment.

An example of what to watch for: On an SOT-23 package, the pad on the side with one lead is made larger than the other pads. This is to provide more force on that side to counteract the tendency of the two pads to want to pull the device that way. In any case, you want to design the pads so that the components can't be pulled so far in one direction that contact is lost on the other leads.

For most components, you can probably obtain a recommended land pattern or foot print to use. If the manufacturer does not have one, often times you can get some guidelines from the generalized package specification.