Introduction: Reworking QFP 120 With 0.4mm Pitch
This montage will show you how I suggest reworking ultra fine pitch (0.4mm pitch) QFP 120s. I will assume tht youa re placing these as part of a prototype build or you have already removed the previous devices and prepped (make sure pads relatively flat at this pitch!) and cleaned.
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Step 1: Place Mini Plastic Stencil W/Repositionable Adhesive
After peeling from the backing carrier (release liner) align opposite corners on the device. Depending on your eyesight you may need to be under some form of magnification.
Step 2: Print Solder Paste
Using a micro squeegee roll the solder paste into the apertures. For this kind of device you can back in forth along each of the (4) sides of the device. Make sure you use the correct solder paste, bring it to temp per mfr's instructions and stir in the jar to get the rheology right.
Step 3: Lift Off the Stencil
Carefully lift off the stencil using tweezers. Grab a corner and lift. Try to apply a consistent upward tangential force as you lift.
Step 4: Now You Have Printed Solder "bricks"
Step 5: Place Device Onto Printed Paste
This is by far the trickiest part of the process. You need steady hands ad probably some form of magnification. I usually use a vacuum tool (making sure ESD procedures are followed) to pick up the device and come straight down gently onto the printed paste. Too much pressure and you will get neighboring leads to become shorted.
After placement I suggest inspecting. If it was not successful here's where you lift the device off the board, clean everything up, and start over.
Make sure you follow the solder paste manufacturer's instructions on the reflow profile. For prototypes a small oven is more than adequate.
Step 6: Inspection
Finally after reflow clean off the flux residue (assuming you are using a water soluble flux) and inspect per the standards you have to meet (Just has to work all the way to Class 3 Space inspection). There you go!