PCB footprint options?

Go To Last Post
10 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi All

 

I know this is a PCB questions, 

 

I have always noticed that PCB footprints from libraries include multiple version for example, ATMEGA48-AU has a 32A_L,32A_M and 32A_N.

 

They have a slight variation on the PAD size of the width and length.

 

Is there a reason for this?  Or this for preference or if you are soldering the component by hand or machine?

 

 

 

 

 

 

 

 

Thanks

Regards

DJ

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

For example, someone one may have an data that suggests that wave vs reflow soldering required a slightly tweaked pad.  An SOT-23 pinout  package from one company may differ slightly from another company & they estimate that their layout should be slightly different to be precisely optimized (versus using the "generic" pad geometry).

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Sun. Jul 11, 2021 - 05:38 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I have noticed that KiCad has many SMT footprints with variants, one for reflow and one for hand solder. The names, in fact, explicitly call out "hand solder" variants. It appears that there is more pad exposed outside of the device electrical contact. I assume that this makes it easier to heat both the contact and the pad with a hand soldering iron.

 

For through-hole (especially DIP), some like oval pads which makes it easier to get a trace between the pads, and some like robust square pads.

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Also: whether the device is expected to be socketted.

 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

ka7ehk wrote:

For through-hole (especially DIP), some like oval pads which makes it easier to get a trace between the pads, and some like robust square pads.

 

I've always been fond of round pads (or even octagons) for my DIPs, but a square pad on pin 1.  Makes life much easier during assembly and test, especially if you have things at weird orientations.  S.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

ka7ehk wrote:
I have noticed that KiCad has many SMT footprints with variants, one for reflow and one for hand solder. The names, in fact, explicitly call out "hand solder" variants. It appears that there is more pad exposed outside of the device electrical contact. I assume that this makes it easier to heat both the contact and the pad with a hand soldering iron.

For boards that will be hand-soldered, I usually extend the pads out in the direction away from the pins, so I can get solder onto the pad and have it flow under and onto the pin.  If the pad is too short, you end up just soldering the pin and hoping the solder flows down to the pad.

Last Edited: Mon. Jul 12, 2021 - 12:03 AM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

it has to do with solder-ability. More specific on how you are going to solder the parts.

If you are only going to do automatic soldering you can use the smallest footprint ( think that will be the _N ( narrow) type) those are a pain in the ass to solder by hand as the pads will hardly stick out from the pins.)

if you are going to do only hand soldering you want the pads to stick out as much as possible to get a good heat transfer and make the chance of soldering a short minimal/smaller. and of you get a short they will be more easily to remove.

if you want to do both you go for a compromise were manual soldering as still doable but a bit harder and you still have a smaller footprint.

 

we always use the Middle size footprint. An ex-colleague made a number of footprints with the smallest pads and those parts are now a hassle to place on our local self build engineering proto. most of the times we tin the pads of the chip, tin the pads on the board, put some flux on the board and use hot air to place those as with a soldering iron we just make a big mess as we cannot get on to the pads to heat them ( QFN package)

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I will also go with the middle size pad, this way it can be done by both machine and any hand soldering when required.

I do not know if this is pad issue, but have found if the pad width is larger then the pin, then sometimes the IC do not sit perfectly in middle but slightly shifted even though pin is still on the pad.

Thanks

Regards

DJ

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

meslomp wrote:
... as with a soldering iron we just make a big mess as we cannot get on to the pads to heat them ( QFN package)
QFN with wettable flanks can be drag soldered.

SMD Soldering - QFN Package - YouTube (7m44s, QFN breakout)

RT8855 - 4/3/2/1-Phase PWM Controller for AMD AM2/AM2+ CPUs | Richtek Technology

 

push technique :

Hand Soldering .5mm QFN Component With SchmartBoard|ez - YouTube (2m45s)

 


QFN soldering tips and tricks - Power House - Blogs - TI E2E Community (hot air)

 

Schmartboard|ez .5mm Pitch, 12 and 24 Pin QFP/QFN to DIP Adapter (204-0015-01) - Schmartboard, Inc.

 

Metcal Blog | Soldering, Desoldering, Electronics Rework Applications, News, Tips and Trends | solder tips

5 Tips for Solder Tip Selection

 

[2/3 page]

Tip #3: Hoof Tips Can Handle Larger Amounts of Solder

[third paragraph]

Hoof tips are useful for drag soldering multiple pins of a surface mount component at once. ...

 

[next]

Tip #4: Knife Tips Have a Useful Edge

...

The slanted edge of the knife tip is also well suited for repairing solder bridges.

...

in-lieu of knife tip, solder flows towards heat (so, a clean and tinned tip)

 

"Dare to be naïve." - Buckminster Fuller

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

gchapman,

I started the first video it has massive pads protruding from under the body those are easy to solder.

Our pins protrude 0.4mm from under the body. that is more than enough for automated soldering, but is a challenge when soldering by hand normally, so we use hot air to put the chip down initially and then with a 0.3mm tip we carefully re-solder each pin to see if it was connected or not.