Issue link: https://iconnect007.uberflip.com/i/1002143
20 PCB007 MAGAZINE I JULY 2018 use it for certain part numbers where they can't approve anything else, but it's a legacy material. There are probably a few military ap- plications where it can't be changed. Goldman: Are OEMs still so picky on the fi- nal look of solder mask? I mean matte versus shiny, semi-matte, colors? Tibbals: Very much so, which always amazes me, particularly when it's buried in a box and no one sees it! In all seriousness, we fully ap- preciate there are some technical reasons why OEMs may favor gloss over matte or specify particular colours. Inspecting gloss boards is hard on the eye. Inspecting matte boards is gen- erally much easier. Likewise, sometimes digi- tal alignment cameras can get tricked by dif- ferent finishes on the board and fluxes or con- formal coatings interact differently with gloss- es or mattes. From a functional perspective, I think there's a fairly well-agreed understand- ing that matte solder masks are less prone to solder-balling during assembly compared to a high gloss. So clearly there are some technical reasons why the finish may be important. The same is true for colour. White solder mask is functional because it's used on light- ing boards for good reflection and black sol- der mask is functional because the OEM may want to avoid reflection or hide track work. So these colours are formulated to meet the particular refection requirements stipulated by the OEM. Although not functional, some end users use certain colors for different re - visions of PCB or to identify prototypes. Over the years we have made pretty much every color under the sun. We probably get half a dozen inquiries a year where people will give us a Pantone number and they'll want the sol - der mask made to that because it matches their logo. Commercially, it makes little sense, but we do it because we keep our customer happy, they keep their customer happy, and the world goes 'round. Green is the historical norm. We have dark green and light green. Light green is better for a solder mask because it makes it faster expos- ing, but people don't often like light green be- cause if they have some stains or marks on the copper circuitry it shows through, whereas a dark green hides a multitude of things on the board. So the standard tends to be a dark green. Traditionally this was always a dark green matte or semi-matte in the U.S. and within Eu- rope. But boards coming in from Asia tended to be a lot glossier and because companies are now trading boards as much as they're manu- facturing, they want some level of consistency in what they sell. As a result, it's now becom- ing quite common for European and U.S. man- ufacturers to want to use a gloss as well. Goldman: What are the major methods of coat- ing, and is there very much curtain coating go- ing on? Earl: The majority of application is by screen, whether that's manual, semi-automatic, or with the double-sided coaters. Spray is another alternative. We see less and less curtain coat - Figure 3: Solder dam profile (50 microns high) of inkjet solder mask. Figure 4: Solder dam profile (100 microns high) of inkjet solder mask.