Awww, don beso tuff on ursef!!!
It sounds like you�re thinking correctly with what you�ve done, but it�s possible that the test you ran did not represent your actual production environment as well as you would have liked. So, you just need to "crank it up a notch", as Eniril says. In doing that, consider this:
1 Run a white ceramic (high purity aluminum oxide) substrate through the reflow oven.
2 Allow the substrate to cool.
3 Print solder paste onto a ceramic substrate in the shape of 0402 pads, using your routine paste, methods, spacing, etc.
4 Place components from both the "old date code" reel and a "more current date code" reel in the paste as you would place components in standard production. Keep track of the components from the two sources. [Just put finger nail polish on the body of certain components or something like that.]
5 Reflow solder the ceramic substrate using the paste manufacturer�s recommended profile.
6 Inspect the solder fillet at X10 magnification.
7 Describe the fillet and the number and size of the solder balls. Ideally, the solder fillets exhibit acceptable attributes as smooth, usually shiny, clearly defined, well feathered (small contact angle), completely wetted metallic bonds between metal surface of the component termination and the solder and there should be no solder balls.
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