With much help from the support forum I'm finally getting pretty good at
creating libraries. One thing is bothering me a bit, though, and I thought
maybe the topic might generate some comments here in this chat forum. Not
being an Eagle related problem, I figured this was a better forum for this
topic.
When creating smd device libraries I've noticed that sometimes the
dimensions given for smd and case size, when carefully followed, would
result in the device pins being slightly off of the pads. This is not always
so, though! Sometimes the pins will be nearly perfectly centered on the
smd's. When I made inquiry to one manufacture why pins were both on and off
the pads when their dimensions were faithfully followed, I was told in
writing that the smd dimensions were "nominal" (i.e. approximate) and that
it was permissible for the smd's to be up to 15% smaller than the device
case pins and so yes, this could happen and it was allowed. I was also
pointed at small print that said somewhat the same thing in the spec. The
fudge was that same small print indicated smd's should, for the device in
question, be the same total dimension as the case (an RK73H series size 2B
device by KOA). But the the case actually overlapped by, yes, 15% when I
followed the recommended smd dimensions. All this was buried on page 69 of a
larger pdf file, by the way.
Well, I've worked in manufacturing for a few decades and one thing I know,
especially when lead free soldering is involved, is that increasing the
amount of solder on smd's greatly impacts warranty returns due to
temperature and vibration environmental issues that cause cracks to appear
in small solder joints. Now, I'm old school where in college they taught me
that solder is never to be used to secure a part, that a mechanical securing
method is to be used instead (this was before smd came along, though I guess
glue is supposed to be the strength when it comes to smd and it's not just
for securing the part for soldering purposes). Also, I know that lead free
solder is harder to make flow properly, thus more subject to cold soldering
problems, and that it's more brittle than is solder that contains lead. I'm
not making a case for lead, I want it out of the environment too, I'm just
voicing what I've come to know over decades.
So why would manufactures create landing specs knowing that maximum contact
(i.e. strength) is being compromised when all available solder area is not
being used? Sounds like a warranty nightmare to me.
