EDAboard.com | EDAboard.de | EDAboard.co.uk | WTWH Media

SMD pad layout

Ask a question - edaboard.com

elektroda.net NewsGroups Forum Index - Electronics Design - SMD pad layout

Goto page Previous  1, 2

Piotr Wyderski
Guest

Tue Jan 08, 2019 12:45 am   



Tim Williams wrote:

> Some call it MSOP (LT?), some call it VSSOP (TI?).

From my experience, MSOP and VSSOP seem to be uniquely defined,
never had a surprise with them. In my case it was 74LVC2G00DP.125,
described in the datasheet as TSSOP8. There is a VSSOP8 variant
too, the 74LVC2G00DC. The "TSSOP8" one is SOT505-*2* -- now compare
it to the typical TSSOP8 footprint from your favourite PCB
design software, which is SOT505-*1*. Then prepare some thin wire
and a good supply of swear words for in-place rewiring.

NXP will never cease to amaze me.

Currently the nice one is INN2904. ~SO20, but not exactly.
One must be very creative in order to use a SO20 adapter
for prototyping.

Quote:
Well, since the holes
are 0.2mm different, and their spacing and diameter are specified with
0.3mm default tolerance...... yeah.


The tolerances of the iTMOV20 varistors are funny too:

https://www.littelfuse.com/~/media/electronics/datasheets/varistors/littelfuse_varistor_tmov_itmov_datasheet.pdf.pdf

Is it at all possible to mount them automatically if the distances
are anything in 2--5.5mm in every direction?

Quote:
Ironically, these didn't actually cause a failure, they just stand out
as particularly horrible drawings.  Most of my actual failures are
attributable to a lack of careful attention to the drawing in the
datasheet (or, when multiple packages are available, picking the right
one -- did that last month, ugh).

ALWAYS check the datasheet!


What for? TSSOP8 is TSSOP8, as it has always been thousansd of times
before. BZZZT!, we got you! :-(

Are they saving letters for a rainy day or what? Why not YSSOP8?
> To answer the OP, this is why no central database is possible.

IMHO a little reflection on "what is D2PAK?" is sufficiently
enlightening, no need for a hunt for exotica.

Placing the reference frame at some random point of the package
is another fancy sport, they do their best to keep your calc-itsu
skills sharp. Always print the footprint and put your part on it.
You'll save a lot of everything.

Best regards, Piotr

Tim Williams
Guest

Tue Jan 08, 2019 12:45 am   



"Piotr Wyderski" <peter.pan_at_neverland.mil> wrote in message
news:q10f32$qle$1_at_node2.news.atman.pl...
Quote:
mkr5000 wrote:

Is there a site somewhere with comprehensive drawings of everything in
SMD packages?

Beware, for one instance there are two completely different packages
referred to as TSSOP8. I have learnt it the hard way, had to re-spin
the PCB. Never trust the packaging specs and check the footprint yourself.


Some call it MSOP (LT?), some call it VSSOP (TI?).

TSSOP (per IPC) tends to be all-inclusive, unless you have a /really/ narrow
package; but it can waste a hell of a lot of board area (i.e., if you take
the MMC pin length plus medium density toe fillet).

There's also 0.5 vs. 0.65mm pitches for various names and pin numbers.

The worst one, no; the worst two I've seen are:
1. An FTDI "QFN" that I stared at the datasheet way too long, and only
determined that it was drawn by an intern. Turns out, it's a *pullback*
QFN, so the pads are actually LGA type -- no fillet. That one didn't ca use
a problem, but the soldering looked weird alright (no outside fillet -- it
was blobbed up in what was supposed to be the toe area).
2. A simple battery holder, CR1220 size I think (or maybe it was 2032), by
MPC. *NO* tolerances specified on the drawing -- only the default in the
title block. So, there's two mounting pegs to ensure you mount in in the
board with the correct polarity, right? Well, since the holes are 0.2mm
different, and their spacing and diameter are specified with 0.3mm default
tolerance...... yeah.

Ironically, these didn't actually cause a failure, they just stand out as
particularly horrible drawings. Most of my actual failures are attributable
to a lack of careful attention to the drawing in the datasheet (or, when
multiple packages are available, picking the right one -- did that last
month, ugh). ALWAYS check the datasheet!

To answer the OP, this is why no central database is possible. Even if
there was, you wouldn't necessarily want it. JEDEC parts for example are
notoriously bad. Compare the DO-214AC drawing to the typical manufacturer's
suggested footprint (e.g., Diodes Inc's). The footprint is utterly
inconsistent with the drawing, because the drawing is consistent with the
JEDEC spec, which their internal package drawing conforms to, but they
aren't going to tell you what their internal tolerances are, because they're
conforming to the JEDEC standard.

And then there's some that are just nonsense, like SFP modules and
connectors. SFP is an open standard and manufacturers make parts
implementing it. They _do not_ specify pin lengths, for example; instead,
they _specify_ the footprint directly! Who fucking knows if anything meets
IPC at all... (And again, the manufacturers do not show their internal
drawings or tolerances, they only repeat the SFP standard they are
following.)

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

Goto page Previous  1, 2

elektroda.net NewsGroups Forum Index - Electronics Design - SMD pad layout

Ask a question - edaboard.com

Arabic version Bulgarian version Catalan version Czech version Danish version German version Greek version English version Spanish version Finnish version French version Hindi version Croatian version Indonesian version Italian version Hebrew version Japanese version Korean version Lithuanian version Latvian version Dutch version Norwegian version Polish version Portuguese version Romanian version Russian version Slovak version Slovenian version Serbian version Swedish version Tagalog version Ukrainian version Vietnamese version Chinese version Turkish version
EDAboard.com map