Homebrew PCBs

On Fri, 26 May 2006 03:06:21 +0200, Trevor Matthews

><trev.matthews@...> wrote:
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>>My next attempt with scrap was going to let the skillet go higher and
>>see what happens, but I've tried to melt it with my soldering iron -  I
>>can move it about and see bright tin underneath, but it doesn't change
>>the consistancy of the crust at all.  I thought I might try to reflow
>>the paste with just my soldering iron and see what happens then as well.
>> Unfortunately, I can't run my skillet much higher than 180C without
>>leaving the lid on, which means I can't see what happens to the paste as
>>it melts, so I can't check to see if the crust is there straight away,
>>or if it forms over time (as it would if it melts, then oxidises as
>>temperature rises and as time increases)
>>thanks for your help
>>Trev
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>OK, if the crust does not melt/reflow when heated with an iron directly,  
>at normal soldering temperatures, i'm pretts sure something is wrong with  
>the paste.
>I can assure you "working" SMD paste does not form a crust, it will reflow  
>with a shiny surface and some flux residue.
>
>ST
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I've tried a couple of things and here are my results:
    1) I placed a couple of blobs of paste on my scrap PCB and melted 
them with my soldering iron - success, the flux burnt off, and nice 
shiny paste.  Even the paste I did not heat directly did the right thing.

    2)  Heated the scrap PCB in the skillet to about 250C.  No change to 
the previous - grey looking surface to the reflowed paste.  Took the lid 
off while it was still very hot and there was a puddle of grey surfaced 
paste with a brown/yellow liquid (?flux) mixed in and surrounding it.

    3) Tried again without the lid on.  Noticed at about 120-150C the 
paste seemed to partially melt - there was small balls of bright solder 
inside the grey muck.  Over the next 20-30C the balls seemed to 
disappear under the grey stuff.  The flux did not obviously activate, no 
fumes. Continued to let the board heat up. Got the board so hot that the 
substrate started to smoulder and the binding glue bubbling out the 
sides of the board.  Still no joy.  Thought I'd try adding my soldering 
irons heat to the process to see what happened.  Flux activated and the 
paste seemed to do the right thing.

  Theory:  Skillet not transferring enough heat to the paste.  Would it 
have anything to do with the board being unetched????

  Don't know how to fix the problem.  Should I try a toaster oven??  
sparkfun.com document lots of problems with melting SMD connectors using 
an oven and I've already tried to hand solder one compact flash 
connector to a board - I'd prefer to reflow it with the other smd 
devices if I could.
 
  What do those experts out there think???

  Trev


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