Just to make it tougher, the joint quality depends on every single parameter of the SMT process. Placement for instance: all QFNs I've seen are fine pitch and an EMS can't visually inspect placement before reflow, unless you as an OEM did your homework and put those corner chevrons in the copper layer.
And then, I have empirically and painfully come to the conclusion that for these parts you can't really use a 6mil stencil, but rather a 5mil. This means that if you put on the same side of the board components with big pads, like coils, you might end up with insufficient solder on those.
And then again, vias in the central pad are tough. Plate them over or dispose them in a pattern that would allow for stencil design that would mask them.
Let's not forget, the tougher the component, the more simple design rules are important. If you want to be safer, you cannot simply throw leadless parts at your EMS.
The good news is that you can quite well inspect them with X-Ray. But you need to do some more homework: what are your acceptability criteria for off placement, solder spread on the pads and voiding. Because reworking these components is a bigger devil and in many cases you are better off leaving a marginal component as is, than having it replaced.
De : TechNet [mailto:[log in to unmask]] De la part de Thayer, Wayne
Envoyé : 11 avril 2008 09:12
À : [log in to unmask]
Objet : Re: [TN] FN Parts--Evidence of Wetting
Saw you at IPC/Apex and was going to say hi, but you looked way too
So, from your response below, the FN parts are treated very differently
from all other SMT parts: Whereas most parts require a visible,
measurable fillet to confirm wetting, acceptable evidence of wetting for
the FN parts a combination of the following:
-an adjacent part with fillets looks like it flowed real good and meets
-the solder on any visible portion of the pads looks like it got plenty
hot and flowed on the pad
-you have documented destructive SPC data on "sister" boards and have
demonstrated/documented good Cpk on all contributing factors
OK, maybe that's acceptable to IPC, but it makes me darn uncomfortable
if I'm dealing with a short run of units I got built at a contract
manufacturer. I guess I'm supposed ask for thorough documentation of
all of these contributing factors so I can confirm I have good joints.
If this is really the case, it seems different enough from the rest of
the IPC specs that perhaps some additional clarification is called for
to explain the note "evidence of wetting". Not only that, but if IPC
really thought that statistics is the right way to get the evidence,
they are smart enough to enter a numerical number for the statistical
criteria to meet. Maybe different thresholds for the three classes.
The idea that the kind of evidence you have described is acceptable for
Class III (medical/military/aero, etc) gives me the creeps because many
of these apps have short production runs (likely not enough statistical
data to make me comfortable). Furthermore, while the original FN
packages always had instructions from the manufacturer to place a
circuit board pad for and solder attach to the die paddle, many
manufacturers are neglecting this today--in fact I have found one QFN
where the data sheet says specifically to NOT attach to this. So the
total stress is just on the pads where the only requirement is "evidence
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