Swapping a Cell Module  - Page 1
Forum section: electronics / general

author: davegravy
author_rank: Regular Contributor
timestamp: February 16, 2024, 01:59:05 pm
content: I bought this sparkfun LTE-M shield:https://www.sparkfun.com/products/14997I want to desolder the SARA-R410S module and replace it with a SARA-R510S, which appears to be pin-compatible.I don't have a good reflow oven, but I do have a hot air rework station.According to theirSystem Integration Manual(section 3.3.6 and 3.3.7) hand soldering and rework aren't recommended.So I guess this is risky, however it's just for one or two prototypes, for the sake of developing software - not a large manufacturing run. On the other hand these modules are pricey.Any suggestions how to approach this?Do I need to use no clean solder paste if I don't plan to clean the board afterwards?Applying heat from above I fear the module's metal shield will conduct heat to the perimeter of the module and result in uneven heating. Should I heat the module from below (there's no components on the bottom)?

author: newto
author_rank: Regular Contributor
timestamp: February 16, 2024, 06:44:46 pm
content: 15 dollar toaster oven from your local thrift store is my suggestion.With the shield on top and the PCB of the module insulating, it'll be almost impossible to use hot air to get the inner pads to melt without completely cooking the chips inside.I've had some success using heating from bottom with hot air to free up ESP32s, and then using low temp solder on the replacement chip, but 5 minutes in a toaster oven is wayyy easier. If you *do* try hot air from the bottom, make sure the board is upright if you want to recover the removed module, or you might find that chips under the shield might come unsoldered and start rattling around on the inside. It's also really easy to bake the underside of the board if you're impatient

author: Kean
author_rank: Unknown
timestamp: February 17, 2024, 11:47:48 am
content: Hot air would be very difficult due to the size of the part. Similarly, using an oven could be quite difficult unless you have fireproof hands (or very long tweezers) as the solder will solidify very quickly after removing the PCB from the oven, leaving you almost no time to lift the part.As the PCB has no bottom side parts, a hotplate will work nicely for both removal and replacement. And as it is just for prototypes (not for sale, no warranty), carefully following a temperature profile is likely to non-critical.

author: tooki
author_rank: Super Contributor
timestamp: February 17, 2024, 01:42:57 pm
content: Quote from: davegravy on February 16, 2024, 01:59:05 pmYou can do this with hot air, by alternately (or even better, simultaneously) heating from both sides, including generously heating the entire board (even the areas far from your module, so they don’t act as a heatsink). An oven is a much, much easier and safer (for the board) way to do this, but you may need to use hot air to keep it molten once removed from the oven.As for external flux: no-clean flux is only safe to leave uncleaned if it’s beenfullyheated. The only way to guarantee this is in an oven. With hot air, you’ll blow molten flux all over the board, but that flux may not ever reach the temperature needed to neutralize it, leaving it in an active (corrosive!) state!(No-clean flux in flux-core solder wire is fine, since it is guaranteed to come into contact with molten solder.)

author: davegravy
author_rank: Regular Contributor
timestamp: May 27, 2024, 11:43:20 am
content: I did manage to do this with a hotplate but ended up destroying the removed module because the module's pcb material is so thin (vertically) and wide (horizontally) my tweezers kept slipping off it and grabbing the metal shield above, knocking internal components off their pads.

author: Kean
author_rank: Unknown
timestamp: May 27, 2024, 12:14:00 pm
content: Oh yeah, I've done that myself - although only on cheap GPS modules.

author: ajb
author_rank: Super Contributor
timestamp: June 08, 2024, 08:05:42 pm
content: Quote from: davegravy on May 27, 2024, 11:43:20 amA lot of the cheap electronics tweezer sets you can find online use a fairly soft steel and can be bent into new shapes (or, as is more often required, back into their old shape) quite easily. So if you need to do this again, grab a cheap pair of tweezers and bend their legs outward near the fulcrum and then bend the legs inward closer to the tips. You want the inside surfaces of the tip to be angled a bit inward (meaning: they are closer together towards the tips) when gripping the module, that will prevent them from slipping upwards under pressure. Too far and they might want to slip off the bottom, but that’s only a problem once the module is free from the board. You can also hit the inside surfaces with a file or, better, coarse abrasive stone, to provide a bit of texture for grip, but the angle is more important.