Solder paste - Black product appear under components - Page 1
Forum section: electronics / projects

author: metRo_
author_rank: Regular Contributor
timestamp: June 02, 2014, 01:03:38 pm
content: Hi,I did solder a board by hand with solder paste and did solder it by parts.Sometimes after solder some components my board get a short between GND-VCC. After search for the problem it looks like it was some black product that appear under capacitors.Per example I solder the mcu and some capacitors and all works ok then I solder some motor control IC and it starts to give me a short between GND-VCC and the problem was on the first capacitors I did solder, not all but one or two. Desoldering the capacitor, clean it and solder it again and all goes well and it works before I need to solder something and heat the board with the heat gun.The solder paste has an year and it was maintained on room temperature against what I'm reading now that I should put it on the fridge.Could the problem come from here?

author: mcinque
author_rank: Unknown
timestamp: June 02, 2014, 01:19:49 pm
content: The solder paste has a tin part and a flux component. Any residual flux on the board can give you problems if it's not 100% electrically inert. Which kind of solder paste are you using? And how about the quantity you spread over the pads? Do you use small quantities?And forgive me for the (maybe) stupid question, but I guess the board that you're using has a soldermask, am I right?

author: Kjelt
author_rank: Super Contributor
timestamp: June 02, 2014, 01:21:36 pm
content: What solder paste do you have? No clean?Solder paste normally has a wetting phase, 40 seconds at 160 degrees the flux flows and works on the pcb and evaporates for a large part, then in the reflow phase the last bits of flux evaporate. If you use this solder paste and quickly solder it with a hand solder chance is that not all flux evaporates and it flows. You should measure with an ohmsmeter if the flux is conductive (hope not) and might be the source of the problem.Another issue could be that you use too much solderpaste and some excess forms small balls between the pins/traces. So you should experiment and look closely under a microscope whats the cause.

author: metRo_
author_rank: Regular Contributor
timestamp: June 02, 2014, 01:38:27 pm
content: The solder is a iso-tip yellow syringe like this one:http://www.iso-tip.com/products-page/smart-paste-series/silver-bearing-solder-paste-7459/The PCB:

author: Kjelt
author_rank: Super Contributor
timestamp: June 02, 2014, 02:10:36 pm
content: From the photo: R14 has way too much solder but U5 seems ok. Can you zoom in on the what you call black stuff? Was the pcb cleaned (with IPA) for soldering?Was the PCB factory finishlayer also leadfree?

author: RiverTown
author_rank: Regular Contributor
timestamp: June 02, 2014, 02:25:09 pm
content: I would not use that paste on electronic.According to the link that is solder paste for soldering metals, who know what kind of flux is inside.Quote

author: Kjelt
author_rank: Super Contributor
timestamp: June 02, 2014, 02:43:53 pm
content: Quote from: RiverTown on June 02, 2014, 02:25:09 pmYep looks definitely not suited for PCB soldering! Good catch.

author: M4trix
author_rank: Frequent Contributor
timestamp: June 02, 2014, 03:08:53 pm
content: What happened to those strip connectors and that jumper? Looks like they are burnt, melted.

author: metRo_
author_rank: Regular Contributor
timestamp: June 02, 2014, 04:03:48 pm
content: Quote from: Kjelt on June 02, 2014, 02:10:36 pmYes, it was made on eurocircuits, PCB Proto optionhttp://be.eurocircuits.com/shop/orders/configurator.aspx.I clean the PCB with Alchool before soldering, there is any special product?From some capacitors that I remove and changed by others:Some more picturesQuote from: RiverTown on June 02, 2014, 02:25:09 pmI asked for help to buy new solder paste:https://www.eevblog.com/forum/reviews/buy-lead-free-solder-paste-syringe-at-europe/Quote from: M4trix on June 02, 2014, 03:08:53 pmIt was me desoldering and soldering again some components looking for the problem.

author: rob77
author_rank: Super Contributor
timestamp: June 02, 2014, 04:46:55 pm
content: if you're soldering with an soldering iron, then simply use a thin (<= 0,5mm) leadfree or 63/37 Tin wire with no-clean flux inside. 0,5mm tin wire is ok - but if you can buy thinner, it's even better.use solder paste only for reflow soldering - either hot air reflow or oven reflow.

author: metRo_
author_rank: Regular Contributor
timestamp: June 02, 2014, 05:00:30 pm
content: I used an hot air gun. The iron was just for small changes.