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

[Mike]

Hi guys, I just finished putting together a prototype board at work that 
had a 100 pin lqfp chip with .5mm lead spacing.  Yes, I used paste and 
hot air from a Weller pyro pen in hot air mode with a 10x stereo 
microscope and then an iron with a 1/32" tip and small solder braid to 
clean it up.  It was alot of work but in the end it did work.  The paste 
I used from Digikey was WAY to thick to get thru any kind of small 
needle.  I just put a blob on a piece of plastic and used a small 
screwdriver to carefully spread it on where the pins go.  If I do this 
again I'm going to try and use a hot plate I have.  Not the kind you 
cook on but the kind with a flat top that you can lay your board on.  
The hardest part by far is getting just the right amount of paste on the 
pins.  Too much and bridges occur, and too little and they don't 
connect.  The paste I'm using btw has been in the fridge at work for at 
least a year and it's still ok.
I asked my board assm. house what they do and they said the people there 
who hand solder that stuff use the dragging a blob across the pins 
method.  I can't seem to get that to work for me.  And it wouldn't work 
if the pins were under the IC of course.

Well, just my $.02
Good luck and most of all have fun!

Mike
KBØozn

Ford Peterson wrote:


> Here is another link from a fella that tried it last night.  Some smoke appeared from the 50 ohm dummy load he was making, but the board looks none-the-worse for wear.
>
> http://www.qrpradio.com/pub/Ham/SDR/SMT/SMT.html
>
> He is using 1206 parts from the looks of it.  I have no problem soldering 1206 parts.  But the technique is amazing.  And he's doing parts on both sides of the board to form a 50 ohm QRP dummy load.
>
> Here is the northrup grumman data sheet on their paste:
>
> http://rocky.digikey.com/WebLib/Kester/Web%20Data/R500%20Solder%20Paste.pdf
>
> Here is the DigiKey catalog page.  
>
> http://dkc3.digikey.com/PDF/T061/1569.pdf
>
> Major OUCH!  $42 without the tip or the plunger for the syringe.  Get this, they must be 2nd day air freight from DigiKey (Air freight from Thief River Falls to rural MN--go figure!)  Must be kept refrigerated too.  I suspect the tip can be used to inject Meth Amphetamines too so they are a bit leary about selling the tips for the solder syringes.  Maybe a diabetic's needle?  No matter--where the heck does one get the needle?
>
> My gut is telling me that this is going to still require a microscope and some decent equipment to clean up the bridges when done cooking.  And I have yet to find anything that shows people soldering down pads on the bottom of parts.
>
> Ford-N0FP
> ford at cmgate.com
>
>
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