Homebrew_PCBs

Hi,

Well I've had the opportunity now to try both the budget hot-air
soldering system using the fish tank bubbler and radio-shack
desoldering system (as seen in the links section) as well as a
Xytronics system that I just got that includes a hot-air pencil.

Firstly, using either system without preheating the board is an
exercise in futility. While the budget system did ultimately melt
the solder paste, it took forever. The Xytronics system never even
got hot enough to melt the paste! Add to that the strength of the
air flow and the parts just flew off the board. So I stopped using
that system for hot-air work. I did ask the vendor about it and they
said the hot-air pencil was really meant for shrink wrap and other
apps and that I should buy the full hot-air rework system if I
wanted to solder with it. No thanks...

Continuing with the budget system, which has a nice gentle flow, I
had very good results on IC's when preheating the board to 150-200F.
That really worked pretty nicely. The tiny caps were still tough
though as they seem to blow away at the slightest provocation.

I also had problems with shorts under caps. Are there any tips about
this? The problem is that the paste starts running and wicks under
under the chip (even a little dab) leading to shorts. With the hot
air, I can't seem to get that to melt effectively under the cap. Any
hints?

For a budget preheater, I got a free-standing heater that looks like
the top of an electric range, maybe 8" in diameter for the entire
unit. I then took a cast iron frying pan, turned it upsidedown and
put it over the entiire unit so the inside of the pan rested on the
heating elements. This seems to provide a nice uniform heat. A
cooking thermometer was used to measure the temperature roughly.

I'd heard the 200F number for preheating before, is this the right
number? Also, how long is it safe to heat the board and components?
From chip specs, it seems like 200F is ok indefinitely, but I'm not
sure.

-Ted

Ted,
I made a Hot Air Pencil and published the idea several years ago at my
web site.
The single most important item is the heat exchanger. In my design I
filled it with stainless steel
wool and that made a vast difference.
Pre-heating the board is required.
http://hans-w.com/surfacem.htm
hansw

Yup, Hans, your page has been in the bookmarks of the Group home
page for weeks!

Hint, hint, everyone, check out the Bookmarks at the Group home
page once in a while. Also a reminder to add useful links.

http://groups.yahoo.com/group/Homebrew_PCBs/links

Do you think one of those warmer plates would work? Not a hotplate,
I mean those glass warmer things. I was just figuring they are
pretty cheap at the thrift store.

Also a reminder that when you do this, the solder paste must be dry
before you start trying to solder, or any fluids will boil and push
the parts off the pads. Should be able to do that by warming it up
on your pre-warmer for a bit.

Steve Greenfield

--- hans@... wrote:

> Ted,
> I made a Hot Air Pencil and published the idea several years ago
> at my
> web site.
> The single most important item is the heat exchanger. In my
> design I
> filled it with stainless steel
> wool and that made a vast difference.
> Pre-heating the board is required.
> http://hans-w.com/surfacem.htm
> hansw

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Hi Guys,

Thanks for replying. I have referenced Hans' links and the others in
the past and made the hot-air-pencil -- that works fine. My real
questions lie elsewhere
Hans indicates that preheating is necessary. ok, one question down.

The outstanding questions are:
What is the proper preheating temperature?
Is there a maximum time you should preheat the board/components
before damage can set in?
What can I do about unmelted solder paste that has moved under
surface mount caps?
What do you do about the tiny cap components blowing away, even under
very minimal air flow? I've read a tiny dab of epoxy. What do others
do?

Thanks,
Ted

--- In Homebrew_PCBs@y..., Steve Greenfield <alienrelics@y...> wrote:
> Yup, Hans, your page has been in the bookmarks of the Group home
> page for weeks!
>
> Hint, hint, everyone, check out the Bookmarks at the Group home
> page once in a while. Also a reminder to add useful links.
>
> http://groups.yahoo.com/group/Homebrew_PCBs/links
>
> Do you think one of those warmer plates would work? Not a hotplate,
> I mean those glass warmer things. I was just figuring they are
> pretty cheap at the thrift store.
>
> Also a reminder that when you do this, the solder paste must be dry
> before you start trying to solder, or any fluids will boil and push
> the parts off the pads. Should be able to do that by warming it up
> on your pre-warmer for a bit.
>
> Steve Greenfield
>
> --- hans@c... wrote:
> > Ted,
> > I made a Hot Air Pencil and published the idea several years ago
> > at my
> > web site.
> > The single most important item is the heat exchanger. In my
> > design I
> > filled it with stainless steel
> > wool and that made a vast difference.
> > Pre-heating the board is required.
> > http://hans-w.com/surfacem.htm
> > hansw
>
>
> __________________________________________________
> Do You Yahoo!?
> Yahoo! Health - your guide to health and wellness
> http://health.yahoo.com

Hi,

> What do you do about the tiny cap components blowing away, even under
> very minimal air flow? I've read a tiny dab of epoxy. What do others
> do?

I prefer solder flux and soldering by hand but if you really want to use
hot air you can use loctite 454 gel cyanoacrylate. It works nicely because
the amount applied is very small.

Best regards,
Alexandre Guimaraes

Steve,
Preheating the board is a requirement, but allowing the flux to "dry"
first is a no no...
Once dry there is not flux and it can't do the job it's meant to do.

Recently I'v seen a strange looking beast for sale at eBay, here's a
link to it.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=1727334195
It appears to apply hot air top and bottom, if this device works, it
would be easy to make...
Anyway, lately I've been using a regular iron, it's simply that much
easier !
I did get a vacuum pickup tool and it helps in placing integrated
circuits, not much use for C's and R's.
When it comes to the really tight packages, I simple flood with solder
and the use the Hakko desoldering tool
to remove the excess and inspect under the Bausch & Lomb at 32X for
shorts...
hansw



Steve Greenfield wrote:

> Yup, Hans, your page has been in the bookmarks of the Group home
> page for weeks!
>
> Hint, hint, everyone, check out the Bookmarks at the Group home
> page once in a while. Also a reminder to add useful links.
>
> http://groups.yahoo.com/group/Homebrew_PCBs/links
>
> Do you think one of those warmer plates would work? Not a hotplate,
> I mean those glass warmer things. I was just figuring they are
> pretty cheap at the thrift store.
>
> Also a reminder that when you do this, the solder paste must be dry
> before you start trying to solder, or any fluids will boil and push
> the parts off the pads. Should be able to do that by warming it up
> on your pre-warmer for a bit.
>
> Steve Greenfield
>
> --- hans@... wrote:
> ? Ted,
> ? I made a Hot Air Pencil and published the idea several years ago
> ? at my
> ? web site.
> ? The single most important item is the heat exchanger. In my
> ? design I
> ? filled it with stainless steel
> ? wool and that made a vast difference.
> ? Pre-heating the board is required.
> ? http://hans-w.com/surfacem.htm
> ? hansw
>
>
> __________________________________________________
> Do You Yahoo!?
> Yahoo! Health - your guide to health and wellness
> http://health.yahoo.com
>
> Yahoo! Groups Sponsor

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