[sdiy] More SMT project pictures

[Tony Clark]

> Yet another dumb question (not that the first was dumb-- I had no 
> idea!)...  If there aren't any thru-holes, then how does one get the 
> component to stay in place while soldering?

   Well in a traditional manufacturing environment the first process is 
to "print" solder paste onto a PCB using a laser cut stencil.  This 
deposits a small "brick" of paste onto each pad where an SMT part goes.  
   The board then moves on to the "pick-and-place" assembler machine 
where the parts are then placed onto the board.  The still soft solder 
paste acts as a sort of "glue" to keep the part in place until the board 
is sent into the reflow oven.
   During reflow, an interesting thing occurs.  The paste turns into a 
liquid (okay we knew it would do that!) and since all liquids have 
surface tension (and the part is now floating on the liquid), the tension 
has the ability to "pull" the part into alignment and cause it to be 
correctly centered over the pads!
   Now often times the above doesn't occur for any number of reasons, 
such as dry paste, too heavy a part (ICs are fairly immune to this 
phenomenon), etc., etc.  Sometimes you'll get a part that "tombstones" 
(ie it's sticking straight up off the board with only one side soldered.  
It happens when one pad doesn't bond well with the part during assembly.

   In super dense boards where components are placed on both sides of the 
circuit board, the SMT parts that are designated to be placed on the 
"bottom" of the board are put on first, but not with paste.  As Harry 
said, a dot of glue is placed under the part and the board is sent 
through the reflow oven to cure the glue.  Once that is done, then the 
top side of the PCB is assembled as described above.
   Once the board comes out of the reflow oven for the second time, the 
top side of the board is now finished, but the bottom still needs to be 
soldered.  The boar is then sent through a wave soldering machine where 
all of the parts on the bottom side are submerged in a bath of liquid 
solder and the board comes out finished.

   Hope that sheds some light onto SMT production!

   Cheers,

   Tony

-----------------------------------------------------------------------
         The E-Music DIY Archive - New Site Coming Soon!
-----------------------------------------------------------------------

"We wouldn't want to ship something that doesn't work"
   - Carl Stork, general manager, Microsoft Windows division
     Excerpt from EE Times April 2, 2001