why don't soldered/brazed joints fail due to dissimilar metals? - Page 1
Forum section: electronics / beginners

author: incf
author_rank: Frequent Contributor
timestamp: April 29, 2025, 10:10:25 pm
content: Why don't soldered/brazed joints fail due to galvanic corrosion/etc.? (or any of the issues that seem to pop with with dissimilar metals in other industries, but not in electronics)Is it just the lack of water? (lack of water doesn't seem to stop other types of joints from failing)edit: Could it be that we ask/demand/expect so little from our soldered/brazed joints compared to other industries that it makes it seem like a reliable process?

author: golden_labels
author_rank: Super Contributor
timestamp: April 29, 2025, 11:30:17 pm
content: There is no dissimilar metals.The keyword you want to search for is: intermetallic layer. Look for microscope photos showing them at soldered joints. Between the solder and the substrate, there form one or more layers. They are made ofcompounds(1)combining both metals. Here’s one example, but you may find better and more interesting photos in your favourite search engine:Source: Reza Sayyadi, Homam Naffakh-Moosavy “The Role of Intermetallic Compounds in Controlling the Microstructural, Physical and Mechanical Properties of Cu-[Sn-Ag-Cu-Bi]-Cu Solder Joints” (Nature, 2019), licensed under CC-BY. (doi:10.1038/s41598-019-44758-3)Even if that wasn’t the case, the joints are sealed. There is no way for electrolytes to seep between the metals, as moisture can’t penetrate there. And without an electrolyte there is no reaction, that destroys joints.(1)Underlining to stress, that this is not merely a mixture. It’s chemically distinct substance, combining elements from both sides. You can tell that, by noticing the sharp transitions. There is no gradient or a lot of tiny blobs, which would be present with a plain mixture.

author: coppercone2
author_rank: Super Contributor
timestamp: April 29, 2025, 11:34:45 pm
content: oh but there is, when they start crackingOn the lowest level I wondered if radioactive decay could be the start point for a crack in a 'perfect' jointAnd if you read about die bonding, you will see that when they start getting into heatsinking dies (die attach), the name of the game is nano-voids or 'bubbles' that effect the thermal conductivity.... formed from soldering process.... its a defect that occurs. So it gets complicated even if you have ultra high control of the joint under optimum conditions with the most care taken. Solder for this purpose is usually gold, and more gold is better. Cheap die attach with high tin and low gold alloy has voids forming, that make the thermal resistance substantial, even if its a 0.001 inch thick 100% solder fill seam.https://www.indium.com/blog/avoid-the-void-identifying-causes-of-die-attach-soldering-voids/Read Indium company stuff to get the nitty gritty on solder. Just don't expect to buy anything from them, even if you work for a small company and your boss wants it.And naturally, even if you are not worried about thermal conductivity, voids are stress concentrators that will be crack formers and generally lead to brittleness, especially for true welds, where you can get hydrogen forming in there... imagine you made a big concrete slab with some air pillows in there you pumped up, easy to imagine what will happen (weaker or failure)And voids are treated along the lines of "less then 0.xxxx% void formation" not "no voids' in a practical process.This one is particuilarly interestinghttps://www.indium.com/blog/avoid-the-void-voiding-in-gold-tin-ausn20-solder-joints/Now, the other solution then adjusting the alloy to account for the plating, for metal soldering, is to tin the thing to be soldered with a iron, then suck off all the solder, clean it, and then solder it. The coating of gold which makes it easy to solder dissolves in the solder, then you remove that, and put fresh solder on, so you have a joint without much gold left. A die is ceramic so they don't have the easy option, if you remove the coating, most solder has a hard time sticking.

author: David Hess
author_rank: Super Contributor
timestamp: April 30, 2025, 02:19:48 am
content: Quote from: incf on April 29, 2025, 10:10:25 pmThey do if the electrochemical potentials are different enough, which becomes very apparent with solders which contain significant amounts of zinc.

author: coppercone2
author_rank: Super Contributor
timestamp: April 30, 2025, 02:46:57 am
content: also look at the aciebo failure for solder weirdness (kind of)

author: IanB
author_rank: Super Contributor
timestamp: April 30, 2025, 04:03:45 am
content: Quote from: incf on April 29, 2025, 10:10:25 pmWhen copper plumbing pipes are soldered with a tin-based solder, there are indeed dissimilar metals in contact with the water inside the pipe.However, I think tin forms a passivation layer that insulates it from the water (that is why tin was used to line the inside of tin cans, and why tin remains shiny even though it is a reactive metal). A more likely source of galvanic corrosion is from impurities in the copper that cause corrosion to happen at grain boundaries. Given time, this will cause copper water pipes to spring a leak. The problem is exacerbated when builders use cheap copper pipes with low quality copper and very thin walls (common in the USA). They install pipes that are certain to fail in the future and walk away from the problem. So yes, galvanic corrosion is a real issue, just not where you might expect.Another place where galvanic corrosion is an issue is (if you live in America) with water heaters. Water heaters have a zinc rod installed as a sacrificial anode. It is necessary to check and replace this rod as routine maintenance every so many years, or your water heater will also spring a leak.