[NLRS] Re: LFCSP (was re:power measurement)

[Dr. Gerald N. Johnson]

On Mon, 2006-02-13 at 14:52 -0600, Ford Peterson wrote:
> Jerry wrote:
> 
> > I've thought of a heat source for soldering these chip scale pieces to
> > boards. Remember the solder sucker irons with a hollow tip. Put a
> > ceramic sleeve around the outside for protection and blow dry air or dry
> > nitrogen through the hollow tip. It should be hot enough to melt solder
> > paste and so solder the chips to the board.
> > 
> > 73, Jerry, K0CQ
> 
> Jerry, Chris,
> 
> I have been following a thread on another reflector that is very promising for purposes of dealing with these itty-bitty parts.
> 
> In fabrication, solder paste is apparently used to secure the bits to the board.  The act of heating causes the flow of the solder and surface tension can cause misaligned parts to snap into place (or out of place I suspect) and solder the connections.  In an oven, the entire board is heated and soldered all at once.  Then I found this just this morning...
> 
> I'm on the Softrock40 reflector, which is dealing with the software defined radio topic.  These guys are doing a SDR using a single tiny PC board with a USB port connector.  Very nifty.  But one of the fellas came up with the following link, which describes using solder paste and a convection oven available at target for $49.  Solder paste everything to the board--both sides I might add--set the oven to 350o and watch the solder flow.  The paste turns shiny at about 335o, at which time you unplug the oven and open the door.  Very nice!  I'm anxious to find a source for solder paste to try it.
> 
> http://www.seattlerobotics.org/encoder/200006/oven_art.htm
> 
> The example describes using the method down to 0.050" parts.  My real question remains whether the method can be used on parts that need to have solder pads under the part--suggesting that the temperature of the part needs to exceed the solder paste melt temp.  
> 
> Anybody out there that works with this stuff on a regular basis?  I would love to pick your brains...  For example, will this only work with a board that has a proper solder mask?  Or will the solder wick up around the pads and legs to form voids between pads, etc?
> 
> Ford-N0FP
> ford at cmgate.com
> 
> 
Worth looking at some more. I don't find solder paste in Mouser or
Allied. Haven't checked Digi-key, they won't let me have a full catalog
and the little is too skinny to bother opening. Dial up slows their web
page searches. In Mouser there are several hot air soldering stations
starting at about $500 with several K as the limit, along side Weller
Soldering Guns that should have never been made.

The storage life of solder paste is short because of oxidation of the
fine solder particles in the flux. But I bet stuff that didn't meet NASA
requirements would still make ham gear.

Solder masks should be a help, but a lot of tiny work depends on surface
tension of the molten solder to break or prevent solder bridges. And
with the small area of the boards, and the relatively low cost of the
tiny chips, scraping is probably cheaper than trying to do any rework in
the factory.
-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer