PCB surface finish questions (automotive, RoHos exempt, needs high reliability)

[walkeryyj]

Hi all, does anyone have any guidance into what PCB surface finish (and solder) would be best for an application that:

-Is not cost sensitive

-Will be assembled right away

-Needs to have very high part reliability and WILL be exposed to years of continuous vibration + thermal shock

-CAN use lead (IE is RoHS exempt)

-Has lots of small SMD components

-Will never be reworked (potted and deployed to the far ends of the world never to be seen again)


Very much appreciate all input

[Dubbie]

Don't touch ENIG unless you really need it. Gold is known to create reliability issues especially with lead-free solders. Even with leaded solders, gold is never a good thing.

I did not know this Blueskull. What sort of issues does it cause? and can you point to any literature on the topic?

[ConKbot]

Don't touch ENIG unless you really need it. Gold is known to create reliability issues especially with lead-free solders. Even with leaded solders, gold is never a good thing.

I did not know this Blueskull. What sort of issues does it cause? and can you point to any literature on the topic?

Gold is dissolved during soldering process. With some, particularly lead free, solders, the dissolved gold makes the solder joint brittle. This is especially evident with thin bonding lines, such as low standoff QFN pads with thin stencil.

Also, the nickel plating, if not done correctly, can cause black pad issue, which may or may not affect solderability. In the latter case, field reliability will suffer.

To add to this, with ENIPIG and lead free, the palladium acts as a barrier, however leaded solder dissolves the palladium just like the gold. So you still end up with nickel-lead-tin intermetallics, palladium in the mix for no reason.  I've had a minor... Issue... Where I had received populated ENIPIG/SnPb boards with unacceptable soldering, but due to time constraints, just put them on a hot plate, doused with rosin flux to re-flow everything. On some boards,I could watch pads de-wet leaving a quite literal black pad. I had to scrub though with solder wick to "save" the board.

Assembly people were  oxidation   but they used a N2 purged oven and still had problems. I'm about 90 percent certain the board house got sloppy with the nickel bath because the Pd would normally save them with a lead free process.

The board house even took a sample back for XRF, but only gave a report on thickness. Not phosphorus contamination in the nickel.

Changed to ENIG surface from Advanced Circuts instead of the original board house, and components stopped randomly falling off the boards.

Not a "typical" case, but if you can avoid getting un-needed metallurgy in the mix (tin plating because you don't need the shelf life, doesn't add any new materials) by all means do so.

[Gyro]

I would have thought good old 'Hot air solder leveled' would fit the bill for reliability. Probably specify eutectic 63/37 leaded solder rather than 60/40, the joints are less likely to become crystalline on temperature cycling.

[Niklas]

Solder pads
Lead free HASL
Don't go for ENIG, immersion tin or immersion silver.
The gold plating on top of the nickel barrier on ENIG will dissolve into the solder and can cause gold embrittlement over time. With leaded solder the threshold is approx 3 percent by weight, but for lead free soldering there is no well defined threshold. Remember that almost half of the printed solder paste is flux and that gold has a higher density than lead.

Solder paste
SnPb 60/37 (leaded)
or
SAC387 or SAC305 (lead free)
Select a solder paste with either of ROL0, ROL1, REL0 or REL1 types of flux according to J-STD-004.

Components
Be careful about large packages with stiff ceramic material, for instance MLCCs and chip resistors, where the vibration and thermal expansion mismatch can cause failures. Instead of using a single 2512 resistor, split it up into several smaller packages in series or parallel.

[jbb]

Lead/tin HASL is a classic if you don’t need good coplanarity. Do you have BGAs or fine pitch DFN/QFN?

The following is total hearsay, and I could be horribly wrong...

What do people know about Organic Solder Preservative (OSP)? I hear it can give good results if a) the boards are soldered shortly after fabrication and b) if they are handled carefully (no touching the pads)

[Nauris]

My experience from automotive is that for such requirements thick film hybrid circuit board potted in soft silicone gel and enclosed in black epoxy package is what is traditionally used.

[T3sl4co1l]

To put slightly more detail on it: it's the gold that's not dissolved, that's a problem.  Free dissolved gold, in solder, doesn't do anything.  If it can't dissolve, then it will form brittle intermetallics, that are a problem.

Gold may not dissolve, for two reasons:
1. Too quick reflow cycle.  The joint ends up nonuniform.  Basically, there wasn't enough solder, time and temperature, to get it dissolved.  Probably, one side of the joint has partially dissolved plating or intermetallics, forming a stress raiser, leading to peeling failure or solder cracking.
2. Too much gold, solder becomes saturated.  Gold remains, and intermetallics form on the plating boundary, leading to peeling failure; or intermetallics precipitate on cooling, leading eventually to cracking of solder joint/ball.

Example, BGA is better than LGA, because the balls can dissolve a lot of gold.  They need to be kept molten long enough to succeed.  LGA might not be able to dissolve that much.  LGA, and QFN, SON, etc., all use the same type of joint.

Repeat the logic for nickel or other barrier metals.  AFAIK, nickel dissolves very little, and very slowly, making it an acceptable barrier.  It would be used alone, except it's nearly unsolderable due to its surface oxide.

In contrast, copper and silver simply dissolve in solder, and precipitates (on freezing) as an innocuous intermetallic (tending to enhance strength rather than brittleness).  Tin plate, silver plate, HASL and OSP are all attractive finishes for this reason.  HASL may be undesirable for flatness reasons (important with large LGAs, QFNs, etc.), but the others should be good.

Tin plate, silver plate and OSP have limited shelf lives, but as you said the assembly will be prompt, this is not an issue, so these are acceptable.

You may ask your PCB fab about what they recommend, or commonly produce, for military and space applications.  Needless to say, if you're asking a fab that's not ITAR qualified, you're probably not talking to the right people.

Tim

[jbb]

Thanks T3sl4co1l, you've reminded me:

A critical ingredient is clean clean clean. Everything needs to be carefully handled, from the boards to the stencils to the solder paste to the component storage.

I also just realised you may have to go lead free to be compatible with all your components.  I understand that RoHS wasn't meant to cover everything, but a lot of component manufacturers just said "OK, we'll just do RoHs parts" and it's now the standard.