Issue link: https://iconnect007.uberflip.com/i/1527613
40 DESIGN007 MAGAZINE I OCTOBER 2024 important to remember that pattern plating, which is done in North American PCB shops every day, is a semi-additive process. We're just taking it to the next level with mSAP. A lot of things are converging right now. You have the finer-pitched BGA packages that are being developed for multiple markets. at's another thing. It's one thing to have lots of room, but if the component is made for multiple markets, you're stuck with that pitch. You just can't go to a bigger pitch. Another point is that exposure units keep getting better and more capable. If your board shop has a direct imaging machine, you likely already have that 25-micron capability. Some of the materials have come together as well, and we supply those materials. Shaughnessy: It looks like partial HDI isn't really necessary unless you have BGAs with 0.5 mm pitch or less, when it's just impossible to mechanically drill for escape routing. Is that the cutoff? It depends on the shop's capability and the material set. But it's important to keep in mind that whatever the pitch is, you have to have some manufacturing tolerance in there, because things will move during the lamina- tion processes. You'll lose some tolerancing in the drill process, although drill machines tend to be pretty good. But still, you could have a few tenths of a mil registration changes in the drilling process. Again, you need to have some manufacturing tolerances. To get a good board, you've got to have some wiggle room. Kelly Dack: Chris, a lot of our designs come from customers who jump right into fine lines, but there's not always a consideration for the materials, dielectrics, and getting everything to scale on the Z-axis and planar axes. What do designers need to know to get this right? One thing that's important to keep in mind is that to have a good transmission line, you need to have a certain amount of copper thickness. It's one thing to take 3- or 5-micron foil and then do 12-micron circuits. ose aren't great transmission lines. If I have, say, a 25-micron- tall trace, with a 25-micron-wide trace, that's a decent transmission line for high speed. e challenge is now I have lots of areas with what we call "neat resin," where the prepreg tends to bridge the top of the circuits. e resin fills in between. en you've got all this bulk resin, and if the resin doesn't have the right properties, it could be a problem for signal integrity and for reli- ability with Z-axis expansion. It's important for the design community to know that you don't want to use a regular FR-4, because all the data we have on its thermal performance is based on the composite of glass and resin. I have some tricks I show my customers. Tin flash is great, because there's no shadowing, so you can tell whether you're developing these areas, and you don't risk your product. Tin flash is just electrolytic-plated tin. You skip the copper plating process, just go into the tin for 10 minutes or so, pull the test panel out, strip the resist, and now I can measure the trace and say, "Yes, I'm getting my 25-micron track. My imaging process is where it needs to be." No need to cut a coupon out and send it out for SEMs; there's no risk to real live product. Chris Hunrath