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Excluding pads from PCB for large Chip

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Jacob Harris

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I have a chip that has 68 pins at 1 mm spacing (SIM900). I know it is not a really tight package but I have some problems with shorts between pins when I am hot air reflowing. Anyway my thought was that I only use probably 15 of the 68 pins so what if I designed my PCB to not have pads for the pins I don't use. If there aren't so many pads I will have a smaller chance of shorting. I use pads from all 4 sides so I figure it will be fixed down well.
 

Hi,

For sure you can do this.

Mind that unused pins must not be left unconnected.

Klaus
 

Not at good idea and not the best way to solve the problem, with the component being a land grid array you may still get shorts. I would suspect there are no solder dams between the pins of the footprint on the PCB and also you paste deposit could do with fine tuning.
The footprint pads should be approx. 0.6mm this allows for 0.1mm clearance round the pads and a solder dam of 0.2mm, used these in the past with no problems.

- - - Updated - - -

How can the unconnected pins not be unconnected if there are no pads on the PCB?
 

KlausST said:
Hi,

For sure you can do this.

Mind that unused pins must not be left unconnected.

Klaus
Click to expand...

Wait Klaus that is exactly what I am planning on doing. 15 pads 15 connections 15 solder joints and 53 pins left without connection simply resting on top the solder mask. Will this be okay.
 

Hi,

This is not recommended since it also reduces the strength of contact of PCB to component. If you are using proper flux with 1mm spacing between pins then you have very less probability that you short your pins. Use of flux removes any short between the pins.
 

marce said:
How can the unconnected pins not be unconnected if there are no pads on the PCB?
Click to expand...

I am saying I would design the PCB to have nothing underneath the unused pins except solder mask.

I probably put too much paste would be my guess
 

Hi

Sorry for bringing that confusion. There is a word missing..
Mind that unused pins must not be left unconnected.
Click to expand...
Should be:
Mind that unused input pins must not be left unconnected.

****
Besides this it seems like a hobby circuit. You can do anything you like for your own circuit..
For professional PCBs I don't recommend to omit the unused pads...


Klaus
 

Okay thanks everybody. The message I got was don't do it! Instead work on proper soldering.

Thanks again!
 

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