How wide do you make your SMD IC pads?

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I'm more software than hardware, so I'm looking for some advice here. One rule I found at some IC manufacturer was to add 0.1mm to the pin width if the pin spacing was > 0.5mm, but just to use the pin width itself for <= 0.5mm. Comments on that?

This came up in particular because I grabbed from a library a QFP64 footprint for 0.5mm spacing that had the pads at 0.35mm wide, with just 0.15mm between pads. This would be for a chip with a maximum pin width of 0.27mm. That pad width just seemed too wide for me, so I wanted to ask about it.

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Depends on the board manufacturer. 0.15mm is 6thou (about 40thou to a millimeter as a rule of thumb). Board manufacturers will normally quote a track and space - 6thou is fairly common - any smaller and you'll start paying a premium. You should also set you pcb package DRC to check for minimum track and space.

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For non thou speakers 1 thou = 0.001" = mil

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ignoramus wrote:
For non thou speakers 1 thou = 0.001" = mil

Being a USA critter, I naturally want to think in inches and thousandths, but the SMT world has beaten that out of me. Now I see a connector spacing of 2.54mm and think "that's weird".

The Rosetta Stone: 1mm = 40 mil (OK, 39.37 mil)

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6mil pad to pad clearance is not to bad.
You have to also keep in mind that how smaller the pads are the more precise placement needs to be.
Most manufacturers now a days have somewhere a document with the by them preferred footprints.

The same thing goes for pad length. The longer the pad the more board space it takes, but the easier it is to solder as the pad extends more from the pin the soldering iron hits the pad better heating it up better.

btw we use mills and mm how we like. Have to admit that in altium it is just the push of a button to change between mils and mm so live is made easy in that way.

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Space craft have been lost because of in and mm mistakes. Well, actually miles and km.

I usually look at the data sheet. Near the end, near "packaging information" is usually the recommended pad layout. Sometimes I make the pads stick out a little farther from the part to make it easier to see if it's lined up right. I've never had success with those parts with no pins, just solder pads underneath that you can't see or probe to tell if they're right.

The largest known prime number: 282589933-1

It's easy to stop breaking the 10th commandment! Break the 8th instead. 

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Torby wrote:
I've never had success with those parts with no pins, just solder pads underneath that you can't see or probe to tell if they're right.
Apparently need some serious magnification (50x to 100x) for the inspections.
Ref.
QFN Package Mounting Guidelines, AT88RF1354, Application Note (Atmel)

"Dare to be naïve." - Buckminster Fuller

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Pad size influenced by soldering process.

Reflow process = smaller pads

wave soldering process larger pads + solder thieves + component orientation

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Ok. So why doesn't atmel give us a land pattern drawing for the tqfp64? It's hard enough to make these WITH a drawing.

The largest known prime number: 282589933-1

It's easy to stop breaking the 10th commandment! Break the 8th instead. 

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Torby wrote:
Ok. So why doesn't atmel give us a land pattern drawing for the tqfp64? It's hard enough to make these WITH a drawing.
If Atmel doesn't provide that information (I haven't checked their site) you can always use another chip manufacturer's data, e.g. Microchip has this overview (see page 371):

http://www.microchip.com/stellen...

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IPC also publishes recommended footprint dimensions

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Achilles wrote:
Torby wrote:
Ok. So why doesn't atmel give us a land pattern drawing for the tqfp64? It's hard enough to make these WITH a drawing.
If Atmel doesn't provide that information (I haven't checked their site) you can always use another chip manufacturer's data, e.g. Microchip has this overview (see page 371):

http://www.microchip.com/stellen...

Now that's a handy PDF. I kept finding drawings with .5mm pitch. The spacing of the pins in a row wasn't hard, but the distance between the rows was. I'm now moving them all out 0.2mm.

The largest known prime number: 282589933-1

It's easy to stop breaking the 10th commandment! Break the 8th instead.