We have used many types of QFN's and built several footprint test boards 
to study all variables. We use inline x-ray so we get a lot of process 
feedback and therefore also try to control even the amount of tilt we get 
in QFN's and try to maintain a specified solder height to keep the x-ray 
data tight. Open vias are bad news, we now plug and cap all vias. Smaller 
vias would help some, but they will still randomly fill with solder and 
cause either insufficient solder, voids, or tilting of the package. 
A solid cooling plate works best. It should be no larger than the package 
cooling plate, and don't print beyond the edge of the cooling plate. With 
capped vias most variation will disappear. You still need to calculate the 
volume of solder you need to get sufficient fill without lifting the 
signal pins. Gridding the stencil works best, and with capped vias  it 
doesn't matter as much how many grids you use as long as you meet your 
target volume.
Profiles are important. Always sacrifice a board and place a thermocouple 
up into the boards cooling pad. Profile design them becomes a compromise 
between reducing voiding and maybe some graping elsewhere on the board. 
Leadfree has been a greater challenge than the tinlead ones we run.

and don't leap of the bridge if you don't know the sparrow answer.  good 
luck.



Roger Mack  P.Eng. 
Manufacturing Specialist
Parker Hannifin Canada
Electronic Controls Division
1305 Clarence Avenue
Winnipeg, MB  R3T 1T4 Canada 
direct 204 453 3339 x373
[log in to unmask]
www.parker.com/ecd







From:
Alan Young <[log in to unmask]>
To:
<[log in to unmask]>
Date:
02-22-2013 11:21 AM
Subject:
Re: [TN] 48-pin QFN  via-in-pad solder slug problem.
Sent by:
TechNet <[log in to unmask]>



Hi folks,

Please forgive me if this topic does not belong to this group.

Let me explain my predicament.
I'm a PCB layout guy who has been asked to look at a soldering issue we 
are having with a 48-pin QFN 7mmx7mm [EMBER CORP - EM357-RTR], the solder 
slug under the part is not soldering/bonding to the PCB. The pcb footprint 
is made per the manufactures specification with a 5.3mm x 5.3mm solder 
slug pad with 9 vias finished drill size of 0.254mm. There are no 
soldermask features on the component or solder side over the vias.

When the board has been reflowed, all of the 48 pins solder well but the 
slug has not bonded to the pcb. There are signs of solder in the slug vias 
on the opposite side of the board. If the part is ripped off the board, 
the slug is clean and solder free.

I do not have any information about the paste screen setup or the oven 
profile.

The questions I have are;

1 - Any ideas on why this is happening?

2 - Are the vias in the slug causing this problem?

3 - Would soldermaks capping the vias on the component side help resolve 
this?

4 - Would plugging (no capping) of the vias with epoxy help?

5 - Do north African sparrows fly faster then European sparrows (just 
seeing if anybody is reading this :-)

I do have some photos and x-rays of the device if this will help.

Again any help or comments would be greatly appreciated.

Regards,

Alan Young.



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