AVR-Chat

Hey Chuck,

 

If you are going to get into SMD work more than just this once, I would
recommend getting a hot air rework station for do any repair work. I bought
one of these from a Chinese company,

 

http://www.aoyue.com/en/ArticleShow.asp?ArticleID=329

 

It was about $240 at the time. I had a rather expensive PCB made up and I
could not get the AT90CAN128 to programme. All connections where checked and
the other PCB was working so I suspected a damaged device. Using the hot air
pencil I was able to safely remove the device without damage to the board.
Some cleaning with solder wick and re-soldering a new device onto the board
and I was back working so for me this purchase easily paid for itself. I
have since used to 2 or 3 times more. The hot air pencil is also great for
heat shrink installation.!!

 

Another purchase I made from China was a re-flow oven and a solder paste
dispenser. All in the 2 off them cost me around $700 with duty and VAT etc.
Doing SMD PCB's is now much quicker and with some PCB shops offering free or
cheap stencils I am considering this method for past application.

 

As Don quite rightly pointed out, make sure you have a decoupling cap next
to each GND and VCC pin. My friend used to make boards without these and
always wondered why the thing kept failing at odd times. Once he started
using decoupling caps his designs where rock solid.

 

Good luck with the 0603 sizes. They are doable by hand and if you have a
magnifying lamp they are even easier. You'll need a really fine tip
soldering iron and see if you can find the finest solder you can get as this
makes it a lot easier, otherwise you end up with too much solder and it
looks poor (although it will work).

 

 

Good luck,

Dave.

PS. I like the UP484 on your website.! Nice toy.

 

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Chuck Hackett
Sent: 24 January 2010 09:59
To: AVR-Chat
Subject: [AVR-Chat] Soldering SMT Atmega16

 

  

I am going to try a project using the SMT version of the Atmega16. This
will be my first SMT project.

I must admit, I'm a bit nervous at the thought of not being able to pull the
processor out of a socket to replace it - not that I've ever had to in the
past, but at least I knew the option was there :-)

I note that, in the datasheet - near the top where it shows the pin
assignments, it shows the TQFP (gull wing) package along with "NOTE: Bottom
pad should be soldered to ground." with an arrow to a large pad on the
bottom of the package. This note does not appear near the end of the
datasheet where they show the package dimensions.

What is the reason for this pad to Gnd - thermal stability?

In a home environment how does one solder this pad - provide a hole under
the part and solder from the back side? Would thermal/conducting grease
work rather than soldering?

My application does not use a lot of output drive so heat dissipation should
not be a consideration ...

Another issue: In the past I have only used commercial prototyping boards
with the processor already in place. I had never noticed that, in the
through-hole part, there are two "GND" pins. The TQFP package has three VCC
and three GND pins.

I assume that good design requires that all of these pins be connected
(i.e.: connecting one won't work)? Do all the VCC pins require their own
decoupling or will one do (and if so, which one?) ?

I'm looking forward to designing a board with 603 size resistors that are
WAY smaller than my usual 3/8" long beasts. I'll be able to get a whole lot
more in the board area provided by the free version of Eagle PCB :-)

Thanks in advance for the help ...
 
Cheers,

Chuck Hackett
"Good judgment comes from experience, experience comes from bad judgment"
7.5" gauge Union Pacific Northern (4-8-4) 844
http://www.whitetrout.net/Chuck



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