Homebrew PCBs

Trevor Matthews wrote:
> 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
>>>
>>
>>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.
>>
> 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???

Should use a higher temperature for a shorter time to reduce oxidation
problems. Try 350degC.