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Slightly OT -Anyonw know the physical dimensions of a TQFP44

Posted: Fri Feb 18, 2005 1:13 pm
by Caffeine
I need to lay out a board with a 44 pin tqfp package but for the life of me I can't find details on the package dimenstions, pin spacing etc!

Posted: Fri Feb 18, 2005 1:20 pm
by Caffeine
Don't mind me, found it in the extended datasheet rather than the summary datasheet (AVR ATMEGA32)

Posted: Thu Mar 10, 2005 2:32 am
by westfw
Glad you found your answer, but in general you need to be careful.
I recently noticed that there are several different sizes of TQFP64
style package. I'm not sure they're all have exactly that name, but
they're close enough to be easily confused...

Posted: Thu Mar 10, 2005 5:16 pm
by MGP
I've done at least 250-300 PCB's over the years, most of them SMD in the past 10 years. One thing I've learned is that you can never trust a PCB footprint unless you make it yourself. Yes, you can use one you get from your PCB software "stock" library or from someone else, but you'll spend as much time verifying the footprint against the data sheet as you will creating it yourself. It may seem like a lot of work, but over time you'll build up a large library of parts that you can trust and are built the way *you* want. You'll also become very skilled with your PCB library editor which makes creating new components easier as time goes on...

ALWAYS check the manufacturers datasheet for mechanical dimensions and make sure that they make sense. I've found numerous mistakes over the years.

You also cannot count on assigning a component footprint based on the manufacturer's name -- they all call components different names even though they may be physically the same or not. Bottom line, always check the datasheet to make sure of the mechanical footprint you need to use.

PCB design is all about paying attention to details. Take some shortcuts to "save time" and you're sure to screw things up. Been there, done that and got the T-shirt... ;)

Posted: Thu Mar 10, 2005 10:00 pm
by westfw
How important are different manufacturers opinions of pad sizes
for SMT components? I mean, even the default Eagle libraries have
several different pcb patterns for something as simple as a resistor,
depending on whether it will be wave soldered or reflow soldered,
and the pad length for ic packages seems almost random...

Thanks
WestfW

Posted: Fri Mar 11, 2005 12:08 pm
by MGP
It really depends on how you are going to assemble the boards.

In general, I use a footprint that's more compact for boards that are going to be assembled by pick and place equipment. I'll sometimes use slightly larger pads for boards I'll hand assemble (especially if I am going to solder with a soldering iron). I have a hot air workstation, so most often I use the same set of pads I use for automated assembly as the hot air tool works fine on them.

Another factor is how much PC board space you have to work with vs. how many components you are trying to populate. If you are doing a very tight, small PCB then you'll want to use the minimum spacing you can. On SMD boards I often hide the reference designators on the silkscreen since they take up a lot of board space and aren't necessarily needed.

One tip regarding silkscreen items -- I noticed that some of the people who ordered PCB's from Sparkfun had silkscreen on their solder pads. This is a no-no as the silkscreen will act as solder mask and prevent soldering to the pads anywhere there is silkscreen. Always check your parts and parts placements for silkscreen on your pads. A lot of PCB vendors will flag this as an error in your design and hold up your order (they figure it's a mistake). It's less critical for a prototype or hand assembled board but if you're going to manufacture a lot of them it's a potential manufacturability issue.

The best answer for boards you are going to mass produce is to check with your contract manufacturer for their advice regarding pad sizes and minimum component placement spacing. They'll know what works well for them and what causes problems.

In the end, it really comes down to common sense. When you have assembled a PCB check to see how it soldered. Did you get nice fillets on all the SMD joints? Did the through hole parts fit the holes and pad spacing correctly? Over time you'll get a good feel for what works and what doesn't.

A good book I use as a reference is Printed Circuits Handbook by Clyde F. Coombs, Jr. It goes through the entire PCB manufacturing process and give a detailed overview of the "hows" and "whys" of circuit board layout. It's not a cheap book but a great reference for PCB design and how PCB's are made.

Posted: Mon Mar 14, 2005 9:46 am
by sparky
I've found that creating adequte part footprints off of a manuf. datasheet is half the battle when laying out PCBs. Add in recommended solder paste layouts and you've got quite a nightmare.

SFE has built up a custom component library (Protel) over the years. Dunno if anyone can use it but it has cost us 1000s of dollars in experimentation (read trial and error) to create.

-Nathan