signal integrety compramised by using non leadded solder. - Page 1
Forum section: electronics / beginners

author: spacedog
author_rank: Newbie
timestamp: October 27, 2014, 02:05:19 pm
content: Hi,First post here, I'd like to say off the bat that I've been watching the EEV blog for many years and think it's one of the best channels going. fair play to Dave and the community for putting out so many great videos over the years. Although I don't listen to the amp hour as often, I'm a huge fan of Chris Gammel and his contextual electronics videos, without it I'd have never been able to learn KiCAD effectively. You guys rock!I'm writing because I'm seeking advice on testing connections for signal integrity. I'm developing a breakout board for these PH sensor devices:https://www.atlas-scientific.com/product_pages/circuits/ezo_ph.html?the bottom right 2 pins are connected via thick traces to a BNC connector for the probe just beside the module. the manufacturer point out that the modules are sensitive to imperfect solder joins on these connections. They recommend SN/CU unleaded solder, I'm used to my SN/PB 60/40 but I gave it a shot. I found that the joins were dull and not shiney compared to leaded solder despite using tons of flux and a 350C iron. many of the connections using unleaded solder gave inconsistent readings from the PH probe. I reworked them with leadded stuff a couple of times and retested using the probes in a known PH solution to confirm accuracy. This is not an option for production, I'm concerned about measuring that the connections are good without having to take an internal measurement from every channel individually.When I measure a bad connection with my multimeter resistance I get the same readings as a good connection, 0 ohms.I was wondering if you guys knew of a reliable method of testing these connections for signal integrity, or have any experience, recommendations or best practices for soldering signal critical connections.Thanks for your time.

author: wraper
author_rank: Unknown
timestamp: October 27, 2014, 02:29:51 pm
content: Lead free solder is dull, it never will be shiny. As of inconsistent readings. Your solder might contain too active flux which is conductive. Post which exactly solder you do use (manufacturer, part number, photo).

author: ConKbot
author_rank: Super Contributor
timestamp: October 27, 2014, 03:47:14 pm
content: PH probes are very, very high impedance (measuring current flow though glass, IIRC) So anything in the high impedance signal path is going to need to be very clean. How did you remove your flux? The PH meter may justify more than normal cleaning procedures.

author: coppice
author_rank: Super Contributor
timestamp: October 27, 2014, 04:06:11 pm
content: Flux leaks like a sieve, but I wonder if the sensor might also be responding to the pH of some flux residue.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 27, 2014, 05:54:24 pm
content: I wouldn't say signal integrity so much as leakage?I can say unconditionally without a doubt that signal quality (meaning, in the GHz domain) is unaffected by the alloy used in solder joints. I can also say the same for low current signals. There may be some error in the microvolt range due to thermocouple effects, but in this case, I'd be thinking about the leakage.Tim

author: SeanB
author_rank: Super Contributor
timestamp: October 27, 2014, 07:43:13 pm
content: pH probes demand a very clean surface at all connections, you must remove all traces of solder and any contamination, as you essentially have a millivolt signal source with an impedance in the tens of megohms driving the cable. Thus you find a lot of the better probes use PTFE wire and meticulous cleaning during manufacture, and also you need to keep the BNC connector inside, outside and back side clean, dry and free of any flux, oil or any trace of liquid or residue from soldering. Then you will get consistent readings. Preferably the board to the BNC connector will have a guard trace around the pH signal trace, and there you need to keep the board absolutely clean.Often you will find the first amplifier ( generally a CA3140 fet opamp) is in a TO100 can, with the non inverting lead bent out from the housing and soldered to a PTFE standoff, and the probe wire is attached to another standoff with a 1M resistor between them, all floating in air. This is done to reduce leakage through the board.

author: spacedog
author_rank: Newbie
timestamp: October 28, 2014, 06:49:56 am
content: Wow, thanks for all the thought/helpful replies. I had never considered many of the points raised and the effect on current leakage from flux residue.Can you guys recommend a method for measuring this kind of current leakage reliably with instrumentation. A google search shows several academic papers on the subject but few practical methods for testing.

author: SeanB
author_rank: Super Contributor
timestamp: October 28, 2014, 06:14:57 pm
content: Simplest is to use your multimeter. Take the pH meter interface out of the socket so you just have the board. Take a 9V battery and apply positive to the centre of the BNC. Take your multimeter and set to 20VDC range. Take positive lead of the multimeter and put on the BNC shell. Take multimeter negative and connect to battery negative.Read the voltage on the meter, this will be the leakage of the board, using the 10M input impedance of the meter as the current meter. Every volt of current measured on the meter corresponds to 100nano Amps of current leaking through the board.If you have a low reading ( like 0.00V) then the board is not leaking, if it measures close to the battery voltage then the leakage is very high. You can then easily compare the readings from after soldering, after the first scrub with IPS and after the second and third wash and rinse, and after drying the board with warm air.

author: Atlas
author_rank: Newbie
timestamp: October 28, 2014, 09:24:11 pm
content: Hey man, These dudes are right. It's the flux.Wash it off with alcohol or acetone.

author: spacedog
author_rank: Newbie
timestamp: October 29, 2014, 11:34:18 am
content: Thanks guysSeanB your method works like a charm, much appreciated.

author: daqq
author_rank: Super Contributor
timestamp: October 29, 2014, 11:49:49 am
content: QuoteBe wary of acetone or stronger stuff - some plastic components might not like that very much. IPA is also good.