0.2mm LQFP lead width vs 0.25mm pad width


#1

On my first iteration I used 0.25mm pad width for a 48 lead LQFP package with 0.2mm lead width and 0.5mm pitch.
I used a stencil and hot air station and I encounter bridges. I was able to use solder wick to fix it, but I’d rather not have to do that. This is the my first stencil and hot air, so maybe it’s just my technique.

I’m considering to decrease the pad size to 0.2mm for next iteration of the board.
Asking all of you with more experience: Is it a bad idea to decrease pad size from 0.25mm to 0.20mm and hence increase gap from 0.25mm to 0.3mm to prevent bridging?

EDIT:
The part in question is CMedia CM108B (package info on page 22)
https://www.cmedia.com.tw/support/download_center/38
Interestingly, I found a slightly different info (0.27mm lead on page 12) for the same chip here:
https://www.semiconductorstore.com/pages/asp/DownloadDirect.asp?sid=1516868576355


#2

You might want to add a datasheet of your part.
And maybe also your manufacturing constrains. (especially with regards to the soldermask layer, paste stencil and stencil alignment.)


#3

Thx for the tip Rene!
I’ve added datasheets. I got my board from JLPCB.com, but I do not know how to read their capabilities sheet.


#4

I would avoid changing the pad size.

Some possible causes of bridging:

  • Excess solder paste
  • Could be that your stencil is too thick
  • Could be that your stencil wasn’t aligned or stable
  • Uneven heat
    • Solder runs to the first section that flows it, so if you have a ground pin that wicks heat to a plane, you could easily have the paste flow from that pin into one that is only connected to a long, thin trace. Fix this by pre-heating the board and playing the air gun slowly.
  • Too much air
    • Running a hot air gun on high speed can blow the solder paste when it gets to the point where the flux liquefies but the solder is still not melted. Fix this by running at a lower air speed.

#5

OK. I will keep the pad size.
I’m pretty sure stencil alignment was good. I ran the hot air at almost its lowest speed.
However, I’m probably guilty of the other points you mentioned.
Thank you for the wisdom Seth!


#6

Post a screenshot showing the copper layer and resist


#7

This topic was automatically closed 30 days after the last reply. New replies are no longer allowed.