Silkscreen on exposed copper/HASL -- will it stick?

[ebastler]

On a small front panel to be manufactured as a PCB, I want to use white silkscreen over exposed copper. (Or rather exposed tin, to be precise -- the copper will be tinned via HASL.) This is purely for cosmetic effect: I am looking for a white surface with a logo showing as an exposed tin pattern.

I don't want to put solder mask below the silkscreen in that area, because the alignment won't be perfect in production: Even if I define the same logo pattern in the solder mask layer, I would still expect to get unwanted fringes of solder mask showing on one side of the logo.

I am aware that board houses will typically clip the silkscreen in areas without solder mask. I intend to ask them not to clip the silkscreen for this board; this has worked before, in a case where I wanted silkscreen on bare FR4.

My concern is: Will the silkscreen adhere properly (reasonably well?) to the exposed metal? Has anyone actually tried this -- intentionally or accidentally -- and can comment on how durable the silkscreen was?

Thanks!
Juergen

[ebastler]

Found one hint in this thread, which had not come up in my initial search: https://www.eevblog.com/forum/beginners/removing-goofed-up-silk-screen-from-pads/. In that case, it was apparently quite easy to scrape off some accidentally placed silkscreen with a knife blade.

Not promising for what I have in mind... But I am still wondering whether the silkscreen could be trusted as long as I don't take a knife to it. (But keeping in mind that this would be for a front panel; so it would have to withstand touching and the occasional fingernail.) Thanks for any first-hand experience you can share!

[matseng]

I once had a batch of boards with severely displaced silk and as far as I remember it was quite hard to remove it from the tin. I think I just soldered over the silk (enough tin was exposed) since it was too much of a hassle to remove it.

[Bud]

In my experience silkscreen adhered well to tinned pads and it was PITA to scrape it off.

[ebastler]

Thank you, Bud and matseng -- that sounds much more promising!

I realize that there will be variations in process and materials between PCB manufacturers; some may use silkscreen ink that adheres well to metals, some not. But based on your reports, I guess the odds are good enough for me to give it a try. I'll do that and report back once I receive the boards.

[mars01]

[...] some may use silkscreen ink that adheres well to metals, some not.

I think that is the bottom of it. You either ask the manufacturer about the behavior or somehow find the datasheet of the silkscreen ink that they use and hope that this data is included.
Otherwise it's just guessing.

[DerekG]

My concern is: Will the silkscreen adhere properly (reasonably well?) to the exposed metal?

There are a few different types of ink used for silk screening ie a different ink is used for aluminium substrate boards where the aluminium will be exposed.

An acid etch base will adhere very well to the tin.

For wearability - remember you can always spray a clear lacquer over the silk screen.

[T3sl4co1l]

Most fabs optimize silkscreen by removing it over pads (plus tolerance).

In other words, over exposed copper...

Put soldermask under the silkscreen and you'll be alright.

Example: place text on silkscreen, normal size.  Copy the text, set it to bold (precisely, if possible: set it to ~3 mil expansion, or whatever the silk-to-solder alignment tolerance is), and set it to negative.  Make sure there is no other soldermask opening under the text.

I like to do a similar thing, but not so much for appearance as for effect: using negative text in copper, positive text in silk, the text is highly readable, and sits in the depression of removed copper.  It feels flat and level, on 1 and 2 ounce copper fabs.  If this is placed over an electrically important area (say, GND vias), it's an indelible mark that can't be scraped off the PCB without rendering it nonfunctional.

BTW, beware of raster graphics.  I've had fabs optimize that out before -- because it's drawn by pixel, and each pixel is, say, 1 mil across, well below the minimum feature size, even though all of them together is an inch across or whatever.  Converting rasters to traces or polygons, or using vector in the first place, is preferred.

Tim

[ebastler]

Following up on my earlier post, I am happy to report that the "silkscreen on HASL" board came out well for me. I had it manufactured at AllPCB, in their lead-free HASL process. They confirmed ahead of the order that they felt comfortable with their silkscreen sticking to the bare metal. I provided instructions with the order not to clip the silkscreen where it was to be printed over the exposed metal, and they did heed those.

The silkscreen does indeed seem to adhere very well. Only caveat: At the edges of the board, my copper layer ends about half a millimeter inside the board contour. (Not a modification AllPCB made; my layout said so.) The silkscreen was specified to extend all the way to the board edge. In that edge region, the silkscreen has sometimes flaked off .

I don't think this is due to a lack of silkscreen adhesion to bare FR4, as I have used that combination before. I assume that the geometry is the issue: Apparently the silkscreen did not like the "step" right next to the milled edge. Not a big issue, but I will avoid that in the future and either have all layers extend to the very edge, or let them all keep the same distance from it.