Which Surface is Best?

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RoHS has been in effect since, when, 2006? Pretty close to five years now. It’s been around long enough that there’s even talk of follow-on legislation. All of the PCB fabricators have pretty much figured out how to deal with RoHS. There are Pb-free versions of every PCB finish at this point. But, we still get questions about the best choice of PCB finish.

I don’t think industry has selected one PCB finish as the “standard” preferred choice. A lot depends on the application and the componentry being used. For large parts, HASL, leaded or lead-free, is a good choice. It’s inexpensive and works well. For leaded work, HASL still seems to be the most common finish. We don’t see quite so much Pb -free HASL, though.

If you’re working with small geometry parts, then you really need to go to immersion silver or ENIG. The consistently flat surface of those finishes will help keep the small parts on the pads where they belong. The disadvantage of silver is that it requires a little more care in handling and storage. It can oxidize which will make soldering more difficult. ENIG is more expensive, but it tends to work real well and is easier to store. Fingerprints can be a problem though. We’ve seen the oils from a fingerprint essentially etch the gold surface off. Weird.

OSP becomes a viable choice with high-volume, cost critical applications. It used to be very sensitive to storage and handling, but has gotten a lot better over the last few years. We don’t see immersion tin much at all. It apparently is harsher on the environment to produce than other finishes.

All that makes it more understandable that we don’t have one preferred finish. It seems confusing, but really it’s not that different than any other product. There certainly isn’t just one preferred style of tire for all motor vehicles.

Duane Benson
Gotta have those monster truck tires if you live in Kelso

http://blog.screamingcircuits.com/

6 thoughts on “Which Surface is Best?”

  1. Duane,

    We’re looking closely at the (fairly) new Plasma Finish from Semblant. It appears to have better shelf life than OSP yet doesn’t come with the material costs of the metal finishes (ENIG, immersion silver, etc.). However, because it’s a plasma process, it does appear to require strict process control (not to mention additional equipment costs).

    http://pcdandf.com/cms/component/content/article/191-new-products/7879-semblant-rolls-out-plasma-surface-finish

    Here’s the patent info: http://www.sumobrain.com/patents/wipo/Halo-hydrocarbon-polymer-coating/WO2010020753.html

  2. The “fun” part…
    Making choices with economic assumptions.

    I have been quoted on the same design.
    (8 layers , 10 mil holes, 5 mil lines/spaces , 5×8 inches, 1oz copper, 0.062thk, high temp FR4 or equ. , fine pitch BGAs, large pitch BGAs)

    at 500 qty…

    Most expensive ROHS HASL (?) … traditionally the cheapest (not really an option for this board)
    Less expensive (ENIG) .. Less than $6.00 each (some quotes as high as $19)
    Cheapest (with better than 1 year shelf life) OSP! … less than $2.00 each including NRE!
    Built 20 samples of the OSP….. they were great!
    (all within last year)

    Enough to drive a Design Engineer or Process Engineer crazy…

    Pricing today is being driven by market forces as much as technology.
    And those forces are constantly (and rapidly) changing!

    Best practice: Review with suppliers during the design process for best “bang for buck”.
    Don’t assume anything based on experience from a few months ago…

  3. Good to hear the OSP worked well for you. This is pretty much exactly why so many answers come out to: “it depends.” Drives me nuts.

  4. We had to reject both immersion silver and immersion tin finishes for our boards because both tarnish in the prescence of chloride and sulphine gases which may be present in the environments where our gas detection equipment gets used.

  5. I have to say from a real board manufacturer 39 years , that this Rohs is still a mess that someone should have no way introduced anything until fully proven.
    We have a letter from NASA that clearly states no Rohs on boards made for them.
    SAY NO MORE.
    I say make these companies that invent the process come up with the things they tested for and how long, in what kind of environment, Pictures ect…. and types of boards. We contuine to change jobs from one thing to the next.

    AGAIN IT’S STILL A MESS SOMEONE CREATED.

    The dictionary doesn’t even have the word Rohs in it.

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