Recommendations for Bare PCB Finish

Phil
Welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers, also known as consultants for ITM Consulting. What have we got today, Jim?

Jim
This comes from D.G. Our company is now looking into going lead-free. Our products are not very complex with some 25-mil pitch components but no BGAs.

The average board size is 3" x 3". We would like to order bare PCBs with a surface finish that will work with both our current leaded process and lead-free solder processes, once we make the change. We are currently using HASL. What finish do you recommend that will work with either soldering process?

That's a simple question. They all work.

Phil
Yeah, they all do with both processes.

Jim
Most all the finishes are backward and forward compatible.

Phil
Your application and the particulars of your assemblies are going to point you in a direction.

Normally, we're not big fans of HASL because most of the industry started moving away from HASL years ago with the advent of fine-pitch components. However you don't have fine-pitch.

Jim
There are people doing lead-free HASL. We don't like it because of the additional higher temperatures that you're exposing the bare PCB to, but it's available.Obviously, look at OSP. You can save yourself a lot of money, anybody can do it, but there are a number of questions, processing questions, storage questions, but OSP is widely used.

Other finishes include immersion silver and ENIG and they would all work. What works best for you is something you're going have to find out, but they're all backward compatible with your lead process.

Phil
So, what we recommend if you're going with HASL, the most common alloy being used right now are the modified tin copper alloys, as opposed to the SAC alloys.

Jim
Tin copper alloys that are being used widely for surface finishes and wave soldering. So, you really can't go wrong with any finish. It's a matter of cost and availability, and, as Phil said, storage, very important.

Think into the future: are you going to be able to stay with these simple components, or are you gonna have to use BGA components someday, or use finer-pitch components, no-lead.

Those are questions which should all figure in your decision.

Phil
We hope we answered your question. Whatever you do don't solder like my brother.

Jim
And don't solder like my brother.

Phil
Because it could be a real hassle.

OSP is a good option as mentioned. My personal favorite turned out to be matte White Tin. For me the Gold electro flash over nickel was pricey, and the silver was not forgiving on process time.

The OSP is the most robust from my experience, but all of these mentioned do require good vacuum packaging for extended storage, or you need to have a very good FIFO with a controlled life span prior to use.

OSP may require some attention to your oven profiles as well. Talk to the OSP manufacturer for those requirements to get best results.

As Phil mentioned above, your application and your process will dictate which one you end up using.

You missed an obvious point in suggesting changes to PCB material. Using Tg 130 material in a lead-free process leads to increased possibility of delamination - especially for double-sided assemblies. We switched to Tg 170 material as a standard when we switched to lead-free. The higher Tg material works great with the leaded process.