Inge,
Thanks for the link. I hadn't seen that article, but I've read items on the positive effect of good center pad soldering. I like the way they describe the opposite effects that can occur from large to small devices, but basically the solution is the same, segment the apertures for better control.
Are there cases where the center pad finish is poorly solderable? Is this caused be the supplier specing a different finish for the center pad?
Thanks,
Gregg
-----Original Message-----
From: Inge [mailto:[log in to unmask]]
Sent: Thursday, July 31, 2008 2:16 PM
To: [log in to unmask]
Subject: Re: [TN] Use of QFN devices in high reliability applications
Gregg,
what's the issue(s). We have been using QFNs for a long time and without
significant problems. However, as a microwave application guy (sometimes)
I'll give you two winks:
1. Have a hard look at your supplier's QFN center pad finish!
2. Correct solder paste volume for the center pad!
Here is a good microwave page, if you don' t already have it.
http://www.mwee.com/202803709
By the way, it's remarkable with the engineering life. Bumble-bees can' t
fly according to technical calculations. But she doesn' know that and is
flying since millions of years without any improvements. QFNs may be the
same ?
Inge
>
>
> To
> [log in to unmask]
> cc
>
> Subject
> [TN] Use of QFN devices in high reliability applications
>
>
>
>
>
>
> Our team is working on several new electronic devices whose application is
> primary, flight critical hardware for commercial aircraft.
>
> Our initial component selection includes many QFN packaged devices
> (leadless, bottom side termination only) which will be mounted on .063"
> FR4 boards using SN63. The devices range from 20 to 44 terminations.
>
>>From studies that are available on the web, a picture has emerged that QFN
> packages have reduced long term solder joint reliability when compared
> with leaded or some of ball grid array devices. Board thickness, flexing
> and whether the device has a central solderable heat sink seem to have a
> significant affect on reliability. Problem is there are no absolute
> numbers, so for critical applications it seems that thorough testing is
> required. We'd rather not wait until the end of a lengthy test regimen to
> find we've gone down the wrong path. Is there newer information
> abbailable? Two of the studies stated that further testing was being
> performed to support the reliability requirements for aerospace and other
> high-rel applications however I cannot find any updated information on the
> web.
>
> We have decided to replace several of the QFN's with leaded devices,
> however there are no direct crosses for 3 of the devices. One of them will
> require a number of components to create the same function; possibly an
> additional circuit card.
>
> Is anyone aware of information or recommendations that would be useful for
> our assessment? Is there any specific information that would drive us to
> use or not use them?
>
> Thanks for your help
>
> Gregg
>
>
>
>
>
>
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