Castellated Holes - Module Assembly Problems - Page 1
Forum section: electronics / general

author: cburgess
author_rank: Contributor
timestamp: April 11, 2022, 04:20:58 pm
content: Our PCB assembly house has recently moved from hand-soldering some modules with castellated holes, to putting them through the re-flow oven with the rest of the components on the boardWe are starting to get a 10-20% failure rate. Some are obvious such as misalignment where the module shifts and the castellations on the module bridge pads on the PCB.We also suspect that we are not getting proper reflow on the joint. I understand that we won't get a substantial fillet that fills the entire wall of the castellation as we used to with hand-soldering, but a few joints look suspect to my eye. One of more tricky faults we are seeing is a partial short ( i.e approx 5 Ohms ) between adjacent pads. Touching up with some flux cures the problem.Anyone "been there and got the T-shirt?" Our PCB assembly house is insisting there's nothing wrong with their process - but we can't ignore the failure rate.I've attached a photo to illustrate the problem - second pin from the left looks iffy to me.

author: KaneTW
author_rank: Frequent Contributor
timestamp: April 11, 2022, 04:36:32 pm
content: I avoid castellated hole modules, but seems like something that can be fixed with an edited footprint and/or stencil? From the picture the planarity seems to be an issue, but might just be the high magnification.

author: Matt-Brown
author_rank: Regular Contributor
timestamp: April 11, 2022, 05:40:14 pm
content: Hi,We have the same sisues as an assembly house.What we do is modify the stencil adn also ask our customer to change the gerbers. We dont have as much paste under the part and extend the solderpaste layer out onto the board mask. This helps the part sit better andas it reflows more solder flows up the terminals. We do only get between 25 and 50% up though, so class 2.They could also try solder preforms.frtom the look of the joints, they have a reduction on the solderpaste layer (widh wise). I would do same as copper

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: April 11, 2022, 11:16:13 pm
content: The joints look a bit dull to me, maybe could be the angle or the quality of the photo. But even if it is lead free paste, it shouldn't look this dull. More often that not, usually the reflow curve need to be retuned to take into account the largest component on the board like your module (yes the rest of the smaller components will suffer a bit but they will be okay). Other than that, the paste opening can be extended further out and I had a feeling the paste could be not that fresh.// ===Add ===I can also see the pads are too long stretching quite beyond what is needed for reflow soldering. Shorten it but keep the same opening length for the stencil. That way, the paste has no where to go but to move towards the castellated pads.

author: SMTech
author_rank: Frequent Contributor
timestamp: April 12, 2022, 09:05:18 am
content: If you manually place a module and then reflow it, and your placement was not good, these modules in no way self correct, on many modules the footprint has pads that are large and quite close together so even a slight offset is bad.The solder does look dull and lumpy but as its (presumably) lead free, and zoomed in and taken using a device unknown that is hard to judge. I would say its underpasted, if anything it looks like the stencil may have a global aperture shrink making things worse. A common solution I have seen with these modules is for the stencil layer to be adjusted with paste placed out past the outer edge of each pad (making it longer) while at the same time making it slightly narrower underneath the device, this leaves a little "excess" solder to wick up the side of the module during reflow. This is effectively what 48X is suggesting above but only changing the stencil assuming you have room. In my experience I would say your footprint is inline with the ones I typically see so I assume that is what you find in the datasheet.I would say that I haven't seen a module have much or any change on the profile needed to reflow a typical board as they aren't that high mass but that might change with very small ovens with a lower thermal capacity.