Silver solder on consumer PCBs? Really? - Page 1
Forum section: electronics / general

author: TerraHertz
author_rank: Super Contributor
timestamp: December 23, 2015, 09:29:12 am
content: While having a look at an inverter PCB from a Sharp microwave oven, I noticed a silkscreen label:"LFa Sn-Ag-Cu"It's Chinese made, I think. How likely is it they are really using a silver (Ag) alloy solder?The solder seems to melt at low temp similar to Sn-Pb solder. Can anyone suggest an easy way to tell if it's really Sn-Pb solder and the logo is a lie, or if it actually is lead free?I'm just curious.Microwave oven inverters technology is progressing interestingly. I grab them when I find them, to pull apart and watch the development. Still looking for an inverter board that's feasible to reverse engineer and modify to use as the core of a regulated HV supply.This one is frustratingly close. The power section is a simple resonant converter, and the control section uses just three ICs, all are 14 pin. Unfortunately two of them have barely legible numbers (I can't make out all symbols) and the third looks completely blank. There was a thin conformal coating, that made them completely illegible, but that rubbed off with IPA. I think the IC part numbers are laser etched, so don't think the IPA rubbed the numbers off too. They were just very faint or completely missing to begin with.The HV transformer is really interesting, in it's simplicity. Magnetic path is mostly airgap. The HV side is that neat encapsulated Litz wire, there's a lot of it, and it can be unwound like off a spool.

author: wraper
author_rank: Unknown
timestamp: December 23, 2015, 09:47:21 am
content: First of all, the melting temperature cannot be like with Pb containing solder for the simple reason that lead free solders (not containing In, Bi low melting crap) have 217+oC melting temperature. Secondly solders containing silver are widely used, one of the most used is SAC305. Solders which don't contain Silver have higher melting temperature, usually 227oC for SnCu types.Thirdly have you heard about RoHS?

author: Whales
author_rank: Super Contributor
timestamp: December 23, 2015, 09:53:13 am
content: Very nice litz wire. I'm looking forward to seeing them pop up in place of the doorstops.

author: wraper
author_rank: Unknown
timestamp: December 23, 2015, 10:00:13 am
content: BTW lead containing solder usually is shiny but lead-free mostly is dull.

author: MarkF
author_rank: Super Contributor
timestamp: December 23, 2015, 10:04:37 am
content: Silver solder has been used in plumbing applications for a long time. But, this is the first I've seen it used for electronics.

author: stmdude
author_rank: Frequent Contributor
timestamp: December 23, 2015, 10:14:07 am
content: I've used silver-solder for some "electronics" for 20+ years.I typically use it for high-current wires that needs to be attached to either a PCB or another wire in radio-controlled vehicles.Why?Well, the resistance of silver-solder is lower than the Pb stuff, so there's less loss (not terribly important), but more importantly, less heat.I've had old-school NiCd battery packs desolder themselves when using Pb solder, as well as wires unsoldering themselves from the speed-controllers. Ag stuff works much better.Oh, and for those thinking this is overkill, we're talking about peak currents up to about 300A at ~8V, so there's definitely some heat being developed.Yep, far from a microwave inverters use-case, but it's used for some edge-cases.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 23, 2015, 10:15:45 am
content: You guys do realise that a fair amount of lead free solder contains some percentage of silver, right?random Kester solder paste I clicked on:http://www.kester.com/products/product/909HPS-Solder-Paste/

author: TerraHertz
author_rank: Super Contributor
timestamp: December 23, 2015, 10:16:19 am
content: Quote from: Whales on December 23, 2015, 09:53:13 amYou mean the fat litz wire? That's the primary, and the wires in the bundle are loose, though with a twist.The really nice stuff is the thin one, on the secondary. It's a twist of fine wires, embedded in something like urethane. Not a core of wires inside a sheath, the wires are all individually embedded in the plastic. Which is the kind that melts and acts as a flux at soldering temp. It's pretty. All the microwave inverter transformers I've seen use it, but previously none of them were structured so you could just use the transformer as a spool of it. Because it has no outer ferrite, this one you can.QuoteYou mean the big old microwave transformers?Recently about a third of street tossed microwave ovens seem to be inverter types. From my small sampling.It's easy to tell, just by the weight of the oven. No need to open them up and look.Curiously this one has no model number that I can see. Maybe it was on a label that fell off.

author: TerraHertz
author_rank: Super Contributor
timestamp: December 23, 2015, 10:44:58 am
content: This is what I get for being out of professional electronics for a decade. I had never heard of SAC solder.https://en.wikipedia.org/wiki/Tin-silver-copperInteresting.No wonder industrial consumption of silver is exceeding yearly production. Will be amusing to see what happens with SAC solder if the silver-gold price rigging system ever breaks down, and they both pop up to their historically expected values.Also it hadn't occurred to me that silver alloy solder would have a significantly lower resistance than SnPb solder. So considering the likely current in that transformer primary (it's part of a resonant LC tank circuit) I suppose all that solder-bump trace cross-section enhancement on the power sections of the PCB is not just for show.

author: tautech
author_rank: Super Contributor
timestamp: December 23, 2015, 11:02:55 am
content: Quote from: TerraHertz on December 23, 2015, 10:44:58 amI'd say it was more to do with electro-mechanical usage: switches, relays, contactors, MCB's etc in which silver alloys have been widely used for decades.

author: amyk
author_rank: Super Contributor
timestamp: December 23, 2015, 11:06:49 am
content: That "vague" one looks like BA10324AF to me, and a quick search shows that they're usually marked with ink and not engraved, so no surprise that the markings came off.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 23, 2015, 03:25:24 pm
content: Terminology:"Silver solder" actually means silverbraze(a filler alloy melting over 800F/427C). A typical formulation being some Zn and Cu, with more than half (the balance) Ag, and melting over 1300-1400F (the mushy range is intentional; brazes tend to be used to fill gaps). Yes, it bothers me too, but alas, this is what it's called. If you call it silver braze, I don't think you'd confuse anyone, and should prefer to call it that to try and clear up this nonsesnse."Solder" (otherwise unqualified) means a low melting (white metal) alloy, i.e., below that temp.What you have is a silver-bearingsolder, i.e., regular solder, but it's bearing some dissolved silver.Likewise, there are many kinds of silver-bearing solders; for example, Tektronix used to hide some inside their (vacuum tube era) equipment, since the 3% Ag content made it less likely to dissolve the silver metallization on the ceramic terminal strips they used.But among lead-free, SAC is the most likely alloy, yeah.(Similar confusion exists for FETs, where the constant current region of operation is "saturation". We should really just forget about that one, too.)Topical chem joke as your reward for reading this,http://www.lab-initio.com/screen_res/nz063.jpgTim

author: MartinX
author_rank: Regular Contributor
timestamp: December 23, 2015, 05:24:12 pm
content: I think the silver is just in the plating from the circuit board manufacturer, in a very price sensitive consumer item like a microwave oven they would save every cent.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 23, 2015, 05:35:30 pm
content: Many decades ago, Tektronix gear used ceramic terminal strips which required silver-bearing solder to prevent the metalized terminals from separating from the ceramic. Tek actually provided a little spool of silver-bearing solder inside the gear...

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 23, 2015, 07:26:54 pm
content: Quote from: blueskull on December 23, 2015, 06:03:12 pmYes Tektronix was a CLASS ACT back in the day.Last Sunday afternoon I went to a concert in downtown Portland. I used the light-rail commuter system, "MAX". The western-branch MAX line cuts through the southern part of the Tek Campus. Maxim bought the Tek semiconductor fab at the north-west corner of the campus, and I suspect many of the buildings are leased out to others since Tek is no longer in the business of complete manufacture of "heavy-iron" test equipment here in the USA.https://www.google.com/maps/@45.4994591,-122.8213203,1095m/data=!3m1!1e3By contrast a growing campus exists just diagonally across from the Tektronix Campus. Nike Corp (makers of sports shoes, etc.) has an even larger (and still growing) campus.https://www.google.com/maps/@45.5079268,-122.8323913,1548m/data=!3m1!1e3

author: tooki
author_rank: Super Contributor
timestamp: December 31, 2015, 04:53:04 pm
content: Quote from: MarkF on December 23, 2015, 10:04:37 amI have a small reel of lead free tin-silver solder I bought at Radio Shack as a kid in the late 1980s or early 1990s. So it's neither new nor obscure, even in electronics.Of course, at the time, I just had a shitty basic soldering iron that made such lead free solder nightmarish to work with so it sat unused for decades! (At the time, I didn't know a damned thing about solder alloys or tip shapes or even the value of flux, though in all fairness, there was no Internet and YouTube back then, nor did I have anyone to show me how to do it!)

author: KL27x
author_rank: Super Contributor
timestamp: December 31, 2015, 09:41:27 pm
content: Lead solder is also made with silver. I remember the first roll of solder I ever bought was Radio Shack 38/60/2, lead/tin/silver. From what I have read, the 2% silver is supposed to increase the mechanical strength.

author: TerraHertz
author_rank: Super Contributor
timestamp: January 01, 2016, 03:04:20 am
content: Quote from: blueskull on December 23, 2015, 06:03:12 pmIt's that sane, helpful engineering attitude that is why I love old Tektronix and HP gear. Designed by genuinely nice and very smart people, long before the lawyers, accountants, stylists and generally insane, evil-minded creeps and psychopaths took over. I don't like the styling of modern gear, and I don't like the corporate ideology that is behind virtually all of it. I feel fortunate that I can get what I want to do done with old gear, and can just ignore everything more recent. Makes me an old fuddy duddy - I don't care.Incidentally, the story of that microwave oven is now on my site:http://everist.org/NobLog/20151112_planning_vacuum.htm#microwaveWhen (if) I get around to tracing the board circuit, I'll update there.

author: helius
author_rank: Super Contributor
timestamp: January 01, 2016, 03:17:46 am
content: SAC305 is the most commonly used lead-free ROHS solder alloy. Chances that it really contains silver? High, as it is only 3% by weight and the price of silver is low by historical levels.

author: tooki
author_rank: Super Contributor
timestamp: January 01, 2016, 03:28:35 am
content: Quote from: KL27x on December 31, 2015, 09:41:27 pmMy understanding is that silver-bearing leaded solder is used for soldering to silver-plated surfaces, to prevent the silver plating from being dissolved.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: January 01, 2016, 07:40:34 am
content: Quote from: TerraHertz on December 23, 2015, 09:29:12 amFor lightly-etched parts that are hard to read, I coat the top with a white marker, and then I wipe off most of it with a paper towel. You might have to add a little alcohol to the towel depending on how much it dried. The white ink fills the etched part, and once wiped off, you can get lettering that gets close to nice silkscreens.