How are these put onto a circuit board, do they use machines?!? their pins are so small.
Posted on 2003-06-08 16:30:59 by x86asm
You bet they use machines.

The machines use suction to pick up the components. This process is called Pick-and-place. The coordinates for all components are in a text file generated by the CAD system when the board is layed out. (This is the "pick-and-place" file).

You should also know that these parts are soldered using an infrared reflow process. Solder paste is applied to all the pads on the PCB using a metal stencil. The components are then placed on top of the paste, which is sticky and holds them in place (sometimes a tiny drop of epoxy is also used in the center of larger components). Then the board is heated while travelling through an IR oven, so the solder in the paste melts and the components are soldered.

On properly designed boards, with the correct pad size for the package, during the infrared reflow process the package tends to "self-align". The melted solder acts like a fluid cushion on which the package "slides" into position. How does it know when it's in position? The surface tensions of the liquid solder become equal on all four sides of the package. Thus, MINOR alignment errors are eliminated.

But the placement has to be as accurate as possible, to begin with.
Therefore on SMT boards you will often see registration marks, called FIDUCIALS. These are reference points. The vision system uses them together with the coordinates in the pick-and-place file to properly align and place the components. Usually there are 3 of these in 3 corners of the board.

Fiducials consist of an etched copper circle (just like a pad without the hole; can also be other shapes, like square or diamond), around which the solder mask is open.
Typically, the fiducials are about 0.060" in diameter, with 0.060" around them clear of anything else, including solder mask.
The copper making up the fiducials can be left bare, or it can be covered with solder, hot-air levelled. That makes it shiny, to improve contrast for the vision system. Therefore the paste is never applied to fiducials. These have to be clean, to be easily located by the vision system of the pick-and-place machine.

Fine-pitch QFP packages (generally less that 0.025" pitch) usually have their own fiducials (2, in opposite corners of the package), to help their placement, since the pitch is so fine.

I hope this will shed some light.
Posted on 2003-06-08 19:23:42 by VVV
I've reflowed quite a few circuit boards on a simple hot plate, though it helped they were on a ceramic substrate (and not G10).

It is actually quite amazing how much placement error the surface tension of solder can fix. I'm talking about 50 degree askew resistors and caps self-alignning as soon as the solder hit the eutechnic temperature.

The biggest problem was the solder expanding as it heated (I think it was actually the flux inside the beads expanding), so the solder sometimes bridged pads, then the pad with the most solder won the surface tension battle, and sucked the other pad near dry. We'd use a pic to push the solder back if necessary.
Posted on 2003-06-09 22:55:18 by Ernie
Our assembly shop prefers 2 fiducials of the same size for the board. We don't have any addition fiducials, even on the boards with 0.5mm pitch QFP's.

0.5mm QFP's can be hand soldered with a fine tipped iron. The size and shape of the tip play a big role in how the solder behaves.
Posted on 2003-06-11 02:26:22 by eet_1024
is there anybody can put a picture what is this TQFP package looks like? till now i just know what a PLCC package looks like.
Posted on 2003-06-25 07:35:58 by dion
I can solder a TQFP with a soldering iron :)

Posted on 2004-09-30 08:02:24 by >Matrix<