Paste Flux quality specification

As part of our process/quality improvement review, it was raised thru' our findings that its a good mfg practice to monitor the incoming solder paste quality (as part of the indirect materials). We get one-page "outgoing QC" vendor reports with each incoming delivery of paste to us.

One element we have neglected so far (and is missing from the vendor OQC report) are the 3 items of the solder paste flux's : 1) clarity, 2) density & 3) colour.

If we ask our paste supplier to impose this element into their standard outgoing QC procedure, was wondering is there any "standard industrial" quality specs for these 3 items? how about "nominal+/-" measurement reading ranges?

Also, we noted the "solder-ball(powder) sizes" varies greatly between different datecode batch delivery to us. The vendor OQC report conveniently omit the nominal reading values out of their report. Upon checks with them, it was told to us the following powder size distribution is their best control specs :

Type 4 paste, Wt%, 1) -500 mesh (38 micron), typical 3.0% & max 17.2%

Now the question is is there an industrial standard that agrees to our vendor's "best control specs" given to us? If not agreeable, can anyone help share what the industrial nominals powder size distribution ranges be?

Thanks in advance!

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