Soldering ESD-sensitive components without grounded soldering iron - Page 1
Forum section: electronics / repair

author: analogix
author_rank: Frequent Contributor
timestamp: June 16, 2024, 12:45:36 pm
content: I'm about to replace some malfunctioning LED modules (Osram DLR-1414) which happen to be electrostatic sensitive according to the datasheet.So with just a hobbyist setup I'm wondering how I should safely solder the new ones in place without risking damage.Now I do have a grounded wrist strap and a grounded work mat, but my soldering iron isn't grounded at all. Is there a temporary solution I can use for this particular job, or is there no way around it than buy an expensive soldering station?

author: Ian.M
author_rank: Super Contributor
timestamp: June 16, 2024, 01:13:46 pm
content: A battery powered or gas soldering iron can be used, and if held by someone wearing an ESD grounding wrist strap, will be very unlikely to cause ESD, even if not explicitly sold as ESD safe.An ungrounded iron with a mains voltage element is not fit for purpose. Directly heated irons tend to have excessive AC leakage current when hot, worsening with age as the mica or ceramic insulation becomes contaminated with burnt flux fumes, and if any fault develops the whole shaft and bit assembly can go live at any time, and there you are holding the solder, a length of bare metal, with which you frequently touch it!Ungrounded low voltage irons *may* be fit for purpose if they have a conventional mains frequency transformer driving them. The mains leakage current is low enough not to usually be an issue. However anything with a switched mode power supply is highly likely to have a Y filter capacitor (to meet EMI regulations) which if not grounded will let through enough mains leakage current to blow the s--t out of unprotected small MOSFETs, some active RF components and some LEDs and laser diodes. e.g. see post #8 onwards ofhttps://www.eevblog.com/forum/beginners/simple-fet-circuit-for-onewire-protocol-doesn_t-work-as-expected/msg1277272/#msg1277272where a Hakko FX888D + a wall socket adapter that didn't ground it, was blowing up 2N7000 MOSFETs.

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 16, 2024, 04:11:51 pm
content: Since this is hobbyist stuff on the cheap:You could attach a wire to the cooler metal part of the iron that is also connected to the tip. Then ground the end of the wire. How feasible this would be, depends upon the construction of the iron.

author: analogix
author_rank: Frequent Contributor
timestamp: June 16, 2024, 06:22:18 pm
content: Quote from: Ian.M on June 16, 2024, 01:13:46 pmI hadn't thought about that!TheDremel Versatipcomes up as an option for this. Most reviews come up very positive, but then again they're quite superficial. Has anyone here used one?I've browsed through several threads here on gas-powered soldering irons and there are mentions of models by Weller, Hakko, Portasol and others, but the Dremel is easy to get hold of here and doesn't cost an arm and a leg.Besides needing it for this specific ESD-sensitive repair project I can see a gas-powered soldering iron/tool is nice for heat shrink tubing.Alternatively I'm about to order componentsfrom Farnell, which I see also sells various gas powered soldering irons at various prices. Obviously the more expensive brands (Weller etc.) are safe buys, but for occasional hobby use I don't want to invest too much in this, but still don't want something that falls apart after 2 times of use (or is unsafe to use).Quote from: Kim Christensen on June 16, 2024, 04:11:51 pmHmmmm... I thought about this, and checking I couldn't find any continuty between the cooler metal part (shaft) and the soldering tip itself. It appears they're isolated from each other, but after having read your post I decided to check again and indeed there's some continuity there.The continuity/resistance reading on the multimeter fluctuates (probably due to bad contact because of the darkened "heated metal" after years of use), so I tried cleaning the shaft with household steel wool which helped a bit, but although unstable it reads around 1 MOhm between the tip and the metal shaft.On a sidenote I measured around 750 Ohms across the two AC (230V) pins. Is this normal? It solders just fine.

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 16, 2024, 08:01:13 pm
content: QuoteProbably good enough. As a comparison, your wrist strap should have a 1M resistor in it's lead for your own safety. It works just fine since there isn't much current from static.As for AC leakage, it is going to come from the heating element and travel towards the tip. Not the other way around, so the 1M between the shaft and tip is probably not an issue.You could do a test if you're concerned. Measure the AC voltage between the tip and ground with a DMM when the iron is turned on. Be aware that the DMM's input impedance (Usually 1-10M) will drag any real leakage AC voltage down. Then add your grounding wire to the iron's shaft and measure again.QuoteIf the iron's element is directly powered from the AC mains, then that sounds about right (70 watt element) assuming it's got some sort of built in temperature control. (A 70 watt iron, without some sort of built in temperature control, would be too big/hot for small electronics soldering)

author: analogix
author_rank: Frequent Contributor
timestamp: June 16, 2024, 08:44:12 pm
content: Quote from: Kim Christensen on June 16, 2024, 08:01:13 pmYou're right. It measures 1 MOhm between the strap and its alligator clip.QuoteYou mean measure the AC voltage between the soldering iron's tip and directly to the ground pin of an AC (230V) wall-outlet in my house?And then measure the AC voltage between the soldering iron's metal shaft and directly to the ground pin of an AC (230V) wall-outlet?Or (in both cases) to the ground pin of an AC wall outlet through a 1 MOhm (or thereabouts) resistor? I have some crocodile clips I could use for this.QuoteIt's a 20W soldering iron without any temperature control (at least I assume not, as there are no controls, LEDs etc.).

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 16, 2024, 10:23:32 pm
content: QuoteYes. Not sure if the plug on your soldering iron is reversible. If it is, measure when it's plugged in the two different ways to see if there's a difference.QuoteNot really necessary, but might make an interesting comparison. I suspect it'll be the same as to the tip, but if I'm wrong, it'll be "interesting".QuoteThe old Weller soldering irons (Though these had a transformer) had magnetic tips that controlled a contact inside the iron's shaft. When magnetic tip heats up, they lose some of their magnetism, opening the contact, so this was a clever way to make a cheap and effective temperature controlled iron. Plus different tips had different magnets so you could change the temperature by changing tips.Point is, that you can't see this mechanism without disassembling the iron wand. The only other clue to this was a slight click as it cycled on/off.ie: no controls, LEDs etc.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 16, 2024, 10:34:49 pm
content: Did solder many times with an un-grounded soldering gun for about 50 years, and don't recall heaving any problems. Also, I didn't wear any anti-static bracelet ever. However, I might have not met all the possibilities, and all the components out there, I might have been just lucky. Your mileage may vary.

author: Ian.M
author_rank: Super Contributor
timestamp: June 17, 2024, 12:34:19 am
content: Weller Magnastat irons e.g. their TCP series rely on a slug of controlledCurie temperaturealloy bonded to the back of the bit. It is ferromagnetic below the setpoint temperature and nonmagnetic above. It is *NOT* a magnet. A magnet at the end of a plunger in the shaft is attracted to the Curie point slug closing a switch in the handle in series with the element until the bit is up to temperature, when it opens. Change the bit to select the temperature. The digit stamped on the slug is the first digit of the temperature in hundreds of Fahrenheit. Unfortunately the original Magnastat bits are getting hard to find. Weller sell a detachable adapter Curie point slug that allows ordinary non-Magnastat tips to be used, but many believe this compromises temperature regulation due to the much poorer thermal contact between the slug and the tip. Also there were a batch of defective temperature switches towards the end of Weller's Cooper Tools era, which probably led to the retirement of many Weller TCP irons.Soldering guns are a different matter. Any with a Weller style two legged bit have an internal transformer and are low voltage at the bit and isolated. However the details of their construction and wiring can be significant as the back-EMF from the trigger switch can be significant when it opens and breaks the transformer primary circuit, and if not snubbed it can couple via the interwinding capacitance to the tip and damage sensitive components. To mitigate this, ideally the bottom layer of the primary next to the core should be connected to Neutral, and the switch should be in the Line feed to the other end of the primary, also there should be a RC snubber across the switch. Grounding the transformer core is also desirable.Ones with a conventional heating element are no different in principle to any other mains iron.

author: analogix
author_rank: Frequent Contributor
timestamp: June 19, 2024, 11:36:48 am
content: Quote from: Kim Christensen on June 16, 2024, 10:23:32 pmOK, I'm not sure if I'm doing something wrong, or expecting different readings but I find these readings a little strange....I've got a digital multimeter set to measure AC voltages.One test-lead is attached to the ground pin of an AC-wall socket while the other test lead will attach to whatever I'm going to test (soldering iron tip, soldering iron metal shaft etc.).Now, the strange thing is that even with only the first test lead attached (to ground) and the second one to nothing at all I get a reading of around a fluctuating 0.07V AC. I can even unplug that test lead entirely from the multimeter and still get a reading. If I plug it back in and actually hold the test lead the reading goes up to around a fluctuating 0.9V AC, and if I actually touch that test lead with my fingers it jumps up to around a fluctuating 7V AC.Next up is testing the (ungrounded) soldering iron: it reads around a fluctuating 9V AC. Both tip and the metal shaft give the same readings.Flipping the AC plug around the opposite way makes no difference.So how do I conclude with these readings? Will it be safe to use my existing soldering iron to solder those ESD-sensitive components or do I need to buy a gas powered soldering iron (like the Dremel Versatip I mentioned earlier)?

author: Krotow
author_rank: Regular Contributor
timestamp: June 19, 2024, 11:50:11 am
content: You totally overcomplicate things. ANY soldering iron/station with soldering tip grounding is sufficient. You can even connect 1 MOhm resistor to ungrounded soldering iron tip with other end to mains socket ground pin and be fine. Honestly in your case I would never bother about ESD at all.Quote from: analogix on June 19, 2024, 11:36:48 amWasn't laughed so loud for a while. Buy a big hammer and smash your device to smithereens. Then go and buy another one.

author: Ian.M
author_rank: Super Contributor
timestamp: June 19, 2024, 02:51:15 pm
content: Modern multimeters are very high impedance (typ. 10 Megaohms for handhelds) and can give a significant AC voltage reading just from the E-field of nearby mains wiring, with the probe leads acting as an antennae. You'll also get phantom voltages on un-powered mains wiring running next to live wires, and from various circuits. The key to figuring out what is actually going on is to shunt the suspect voltage with a resistor to make a low impedance voltage measurement. As you have determined its low voltage, an ordinary 10K resistor in parallel with the meter will do. If you suspect higher voltages will be present,use a higher value, higher wattage resistor e.g 100K 1W will be OK up to 300V (or the voltage rating of the resistor, whichever is lower). Knowing the resistor and the new voltage reading with it in parallel, you can calculate the AC leakage current and decide if it is unsafe for your components.I suspect that your existing iron although not ESD safe is unlikely to damage those LED display modules. That may not be the case if you used it on more delicate parts, e.g. extremely ESD sensitive RF MOSFETs.

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 19, 2024, 03:26:31 pm
content: I'd say you're safe to just solder the LEDs with that iron. It doesn't have any significant AC leakage. Like Ian.M said, your meter will pick up AC out of the air.But there is no harm in grounding the shaft of the iron anyway.I'm not really a fan of gas powered irons for regular soldering jobs because it's hard to control the tip temperature. Not to mention the hot exhaust/flame coming out if it that can burn other components in the vicinity of what your soldering.I'd only use a gas powered iron in remote locations where getting AC power is impossible and if the job was not too delicate.

author: analogix
author_rank: Frequent Contributor
timestamp: June 19, 2024, 09:11:41 pm
content: Quote from: Ian.M on June 19, 2024, 02:51:15 pmI had no idea about this. Does this mean that with fluctuating/floating voltages like this you can take the reading with a grain of salt? While with "normal" voltage measurements (i.e. connecting the probes directly to a power source etc.) the readings will be more correct without needing to think about having an appropriate resistor in series with the probes?QuoteGood to hear as I'm comfortable with this soldering iron after years of use (meaning less risk to do a bad job).The main reason I'm discussing ESD sensitivity is because a section of the display module's (DLR1414) datasheet (see attachement below) warning about ESD sensitivity.Quote from: Kim Christensen on June 19, 2024, 03:26:31 pmGood point. I've never used one, but what you say sounds reasonable and without the necessary practice I might very well damage both the replacement LED modules and the PCB in question due to excessive heat and a too big tip etc.But the ESD-sensitivity warning in the datasheet has got me worried. It's hard for me as a non-engineer to know if I should take this very seriously or if they're bound to exaggerate and point out all the "in the worst possible case scenario" type things to avoid getting sued or whatever if they hadn't done so.So if I use my regular soldering iron, would you recommend I find a resistor (around 1M Ohm, but I understand that's not a critical value) between the shaft and the ground pin of the nearby AC wall socket?

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 19, 2024, 10:35:01 pm
content: No need for the resistor. You can just ground the shaft directly which is what is done in commercially made irons. By not using a resistor, you'll actually make your iron a bit safer.ie: If it had an internal failure and the element shorted to the casing, then the tip would be live at mains voltage if you'd used a resistor in the ground lead. But with a direct connection, the breaker in your house would have tripped alerting you to the danger.The 1Mresistor in the wrist strap is foryoursafely. What it does is limit the current flow through your body (via the wrist strap) if you accidentally touched a live wire while wearing the strap.Touching a live wire is never a good idea, but if your body was completely insulated from the ground (current return path) then nothing would happen to you. But wearing a static wrist strap provides a guaranteed path to ground. Hence the 1Msafety resistor.

author: Ian.M
author_rank: Super Contributor
timestamp: June 19, 2024, 10:39:15 pm
content: Personally I get good results with a catalytic gas iron, a literal ''firestick', even with fairly fine pitch boards. However as it *is* a 'firestick' with no temperature control, albeit variable power, and there is quite a lag between adjusting it and the gas flow change taking effect, one needs considerable experience and skill to avoid overheating stuff. Probably on balance if you need a portable mains-free iron, the less experienced should get a low voltage temperature controlled one, possibly powered by a USB C power bank or a common power tool battery so you can easily swap battery packs if you run out of juice at an inconvenient time. Do not get suckered into buying anything less than 40W max. power to the tip at the nom. battery voltage you intend to use.Not a resistor in *series* with your DMM, a resistor in parallel with it! i.e. to make a low impedance voltage measurement, put the resistor across the points you want to measure voltage between then apply the probes.A mains soldering iron bit should be hard grounded - as close to zero ohms*to mains PE as you can manage. This may be problematic in some parts of Norway where the shallow soil depth and underlying rock type makes getting a good ground for the mains supply difficult, so you may not actually have a grounded socket. In that case, it should be grounded to wherever your ESD ground goes#, but without any series resistor. It may be possible to refit your iron with a three wire mains cord and three pin grounded plug, or you may need to use a small metal spring clip on the shaft of the iron to connect the ground, as close as possible to the handle so it doesn't get too hot.* Ref:NASA-STD-8739.3, section 6.4.3 (page 30):Quote# Do *NOT* ground to a water pipe in a multi-household building. Unless you control all plumbing from the point of entry to the point you wish to ground to, you cannot guarantee its integrity as a ground which may have to carry a fault current. It is unethical to risk a fault in your equipment making someone else's pipes mains live!

author: wraper
author_rank: Unknown
timestamp: June 19, 2024, 10:50:08 pm
content: Don't bother, it will be fine. Unless you start manufacturing in volume, very unlikely you'll ever see any of them fail because of lack of ESD precautions.

author: analogix
author_rank: Frequent Contributor
timestamp: June 23, 2024, 07:34:01 pm
content: Quote from: Ian.M on June 19, 2024, 10:39:15 pmI completely agree, but I believe it's mandatory to have "earth circuit breakers" installed in every home's electrical installation -shouldn't that prevent situations like that if AC voltages from the soldering iron accidently goes to ground?As far as I know,currentbuilding regulations demand that all AC sockets are to be grounded innewbuildings, but that's not the case with older housing, so in my case there's grounded AC sockets in the bathroom and kitchen, but not elsewhere. So I'll have to use a grounded extension cord, then attach a wire with crocodile clips at each end between the soldering iron and the extension cord's ground pin.The soldering iron is plugged into an (ungrounded) AC socket where I have my work desk.UPDATE: success! I did my best with the ESD-protection and went ahead with the desoldering/soldering -and the new display is working perfectly!I grounded the ESD-mat and wrist strap, but didn't do anything with the soldering iron, but it might be good to learn what I should do the next time for the ultimate in ESD protection (I suppose that would be to buy a proper ESD-safe soldering station and all).