Question: Why are there specks present in foil or metallized base laminate? The structures we see them in are: PET/foil/PE or BOPP/foil/PE.
Answer: There is a lot you have left out of your question so a definitive answer will not be possible. First off you don’t give a description of the specks (clear, dark, brown, larger than the layer, which layer they are seen in etc). You really need to isolate the layer and the nature of the specks to determine the real source. And once you do this identification the source is generally easily determined.
But looking at the two structures what is common to both is the foil and the PE layer. There are two principle possibilities for the specks.
First it is possible that the foil is dirty and that the foil surface is covered with dirt and debris. Then once the laminate is formed the specks are visible on one or both surfaces of the lamination. Use a tack rag to clean the foil as it unwinds and see what you pick up off the foil as it unwinds.
Second, the PE layer could be the source of the specks. This could be due to excessive “gels” in the resin due to poor filtration in the polymer plant, contamination of the PE resin when charging the extruder (dirty box, heavy dust in the resin handling area, dirty resin system), poor selection of PE grade for the application or the extrusion conditions are forming the specks in the extruder and melt system. You can get an idea as to the potential source form an examination of the specks. You can get quite a bit of information about the specks with a 30X to 100X magnification using a light microscope.
Using the microscope I have identified specks in various laminate and film products from wood particles in the resin, hard black CaCo3 specks on laminating paper surface, clear cross-linked gels in LDPE, light brown gels, carbon particles in LDPE layers due to poor screw design, unmelted pellets in PP or LLDPE layers due to poor screw design or process settings, etc