Surface mount with very tiny parts at home

Thread Starter

PickyBiker

Joined Aug 18, 2015
88
I have a tiny chip, Si4744 with a QFN 24 footprint. There is no way to solder this to a breakout board by hand. I am looking for the best way to solder this to a breakout board that has copper pads that are not tinned.

Here's what I have

Si4744 Radio chip
QFN 24 -> DIP breakout board with no solder
Oven with accurate temperature sensors
Kester solder paste (really thick stuff)

I don't know a way to put the paste on the tiny pads without it filling in between the pads. Since the Si4740 pads seem to be tinned, I thought about trying to just put some flux on the board and heat it up to solder it without a paste. Balking on that because these chips are $23 apiece and I don't want to damage it.

Here is the datasheet for the chip
http://www.silabs.com/Support Documents/TechnicalDocs/Si4740-41-42-43-44-45.pdf
Here's the datasheet on the breakout board
http://www.chipquik.com/datasheets/PA0064.pdf

Ideas for doing this will be appreciated.
 

tracecom

Joined Apr 16, 2010
3,944
Tin the pads, and then use a stencil to apply the paste. Reflow.

My guess is that there are QFN24 stencils on ebay, but I haven't looked.

ETA Now that I think about it, I have soldered QFN24 packages without a stencil. I'll look for my records of how I did it.
 

Thread Starter

PickyBiker

Joined Aug 18, 2015
88
I decided to try it. Under a digital microscope I put a thin layer of paste and then meticulously cleaned out between the pads. It took quite a while, but it worked. If I have to do that again, I am going to look for one of the solder masks you mentioned.
 

AnalogKid

Joined Aug 1, 2013
8,251
If the layer of paste is thin enough, doesn't the surface tension pull it to the pads once it melts?
Yes, and it is cool to watch. Crooked resistors line up in neat little rows. But "thin enough" is a bear. We use 6 mil thick stainless steel stencils for normal SMT, and maybe thinner ones for the tiny parts like 0204.

ak
 

SLK001

Joined Nov 29, 2011
1,543
The layer of solder has to be thick enough to "float" the part on the molten solder. Too thin a layer and the part won't float, thus no pulling it straight.

I have soldered QFN parts like this using only a soldering iron and a thin tip. Since the pads are all on the outside, it is fairly easy. If the pads aren't tinned, then tin them, then wick all the excess away (my old company used to get boards with the pads ENTEC'ed, which was CRAP in my opinion). Apply paste flux on the pads. Place the part on the pads. Apply solder to the tip of your iron and touch the tip to the pad on one corner. Make sure that the solder bonds to the QFN pin. Repeat on the OPPOSITE corner. Once secured, solder the remaining pins one-by-one.
 

tracecom

Joined Apr 16, 2010
3,944
I decided to try it. Under a digital microscope I put a thin layer of paste and then meticulously cleaned out between the pads. It took quite a while, but it worked. If I have to do that again, I am going to look for one of the solder masks you mentioned.
Good for you.

Just this afternoon, I removed an SOIC-16 that I thought was defective and replaced it with another. I applied some flux, placed the new IC, and wiped all the pins on one side with solder. Then I applied another blob of solder and wiped the pins on the other side. A quick soak on each side with some solder braid and all the solder bridges were gone. Worked like a charm.

SMT ICs are surprisingly heat tolerant.
 

SLK001

Joined Nov 29, 2011
1,543
SMT ICs are surprisingly heat tolerant.
Not any more heat tolerant than their thru hole brethern - actually, they are LESS tolerant, since they are smaller. A PDIP package has a max temp of approx 260°C, while a SOIC package has a max temp of approx 220°C.
 
Top