High thermal mass desoldering "pliers" - Page 1
Forum section: electronics / general

author: forrestc
author_rank: Unknown
timestamp: June 16, 2022, 11:45:08 pm
content: On a somewhat regular basis, we need to remove and replace a header on a production circuit board. Lead-free, SAC305.So far, I haven't found a reliable and consistent method. Yes, I've tried all the "normal" methods like solder wick, desoldering iron (pace SX-100), heating the solder side with an iron and pulling it with pliers, etc. The problem seems to be thermal mass combined with the irritating behavior of SAC305 when reheated.I would love to buy a small solder fountain for this, but that has its own issues, especially surrounding copper dissolution issues and the cost of filling yet another pot with SAC305 or similar.I think that a very workable solution would be if I could actually grab the pin itself on the connector/non-solder side with the soldering iron. Then I could have the iron heat the pin to the solder melting point, which would permit me to pull the pin out of the board.That is, an iron which is basically the same form factor as SMD tweezers but with small needle-nosed plier ends as the tips. And, of course, a bit more wattage than your typical SMD tweezers since you're heating a relatively large pin that may be attached to a ground plane.All of the SMD tweezers I have aren't suitable for modification to this. They are either too low power or have an integrated heater in the tip, and so on. Also, many of these have a large gap between the tips, so it would be difficult to modify for grabbing.I'm posting this here because I'm hoping someone could point me toward either a product that already exists OR a tweezer that has enough power and a mechanical design that could be modified to have some sort of 'grabbing tips' installed.

author: thm_w
author_rank: Super Contributor
timestamp: June 17, 2022, 12:27:21 am
content: Photos?What iron and tip were you using to heat the pins?Are we talking about a normal 0.1" pin header that you want to entirely remove?Preheater would help.Hot air with an appropriate nozzle to cover a rectangular section of the header might work.If the header is not that big, a wide flat tweezer might be able to heat all rows of pins.I don't think the tweezers being able to grab the pin is going to be any better than the same tweezers in combination with pliers to pull the pin. Depending on what kind of header we are talking about, if it has the plastic shrouding.

author: forrestc
author_rank: Unknown
timestamp: June 17, 2022, 02:24:20 am
content: It's a combination of 0.1 square headers in various arrangements such as 3x5 and 2x20 and also multipin .156 pitch headers.Note that these are selective soldered connectors, typically with very good top side fillets. We tend to prefer a smaller hole to pin ratio (while still having enough room so that the solder wicks properly), so that the connectors end up more precisely aligned. This lack of clearance seems to be at least part of the reason why this is challenging.There's also some RJ45 connectors including a big 2x4x8p8c block, but I'm not considering this method for those. Plus these rarely are an issue.Using a Pace SX-100 with an appropriate tip (have tried several), I don't consistently get enough solder out from around the pins to have the connector release. I have replaced the handpiece, verified good seal and suckage and so on. Obviously, some pins are fine, but enough don't that it isn't really a useful method. I've also wondered if a different solder extractor (more vacuum?) would do better, but have not found any quality comparisons between what I have and what is out there.Attempting to heat the solder side with a pace TD-200 iron with high-performance tips successfully melts the solder, but pretty much as soon as you grab the pin with a small needle nosed plier, enough heat is drawn off the pin to re-solidify the solder in the barrell. This is why I'm figuring that using a hot tweezer that can pull might be useful here. FYI, I've tried various temperatures up to what is beyond reasonable for rework. Plus, when this method is successful, it seems to like to lift the top pad fairly regularly (scrapping the board). I think what happens is that the solder on the top pad solidifies around the pin fairly quickly - again something avoided by using a hot "needle nosed plier" type of device.I've also attempted various preheat on the board. Obviously, getting the board up to just below liquidus would be ideal, but I can't go that hot due to the melting temperatures of some of the connectors. I don't remember what the hottest temperature we tried was. But it didn't help a lot. Plus, there's the whole issue of can't really keep the preheat on what you want while you're soldering since if you preheat from the connector side you end up with the board under the connector being shaded, and if you preheat the bottom the board starts cooling off immediately once you flip it to do your operation. (I might be missing some strategy here)On the .156 header, I've had fairly good results with a "heat all pins at once" method, but the widest tip I have is just under the width I need, and this doesn't help with the 3x5 headers. The 2x20 headers are too wide to attempt.I have tried the lead-free chipquik alloy but don't seem to be able to get enough mixing to reduce the melting point.And yes, I've tried various fluxing alternatives, including squirting some flux onto the top fillets with a syringe where they are accessible.Note that all of the methods above I've used before with leaded solder. Because these go to RoHS countries, we have ended up needing SAC305. Fortunately we don't generate enough rework that this is a giant deal, but we'd really like to be able to recover from headers which for one reason or another need to be replaced before shipping. Which is why I keep looking for alternatives.

author: thm_w
author_rank: Super Contributor
timestamp: June 18, 2022, 01:34:53 am
content: I'm not sure what power or capability SX-100 has, so I can't comment there. If the holes are undersized that will probably make it harder as you say.Another alignment method is tooffsetthe holes.I'm not a fan of the ADS200 for high thermal demand applications. But, from what Shock stated, cranking the temperature should compensate.If you have access to a T12 station, you could try ultrawide tips like this:https://uk.rs-online.com/web/p/soldering-iron-tips/5175575Metcal tweezer tips available up to 28mm, or regular tip up to 45mm wide, but its not cheap:https://www.farnell.com/datasheets/328503.pdfFor RJ45 I've only had luck with hot air, let it sit for a few minutes away from the board and soak (I use a flexible arm to hold the hot air gun in place), then bring closer and wiggle the connector out. The plastic in the connector will be distorted and melted by the end.You may consider some thermally insulating pliers, maybe stainless or ceramic if they exist? To prevent the heatsinking. Not sure how strong they can be though. Or wrap the tips in silicone tape.