Hand placing difficult components - Page 1
Forum section: electronics / general

author: FR4
author_rank: Newbie
timestamp: October 17, 2023, 07:44:06 am
content: Hello,I've made a board with SMT components, sized for hand placement. I can reliably & quickly manage 0805 and 0402 with tweezers, and have the odd small IC including a 12pin TQFN (2mm x 2mm) which is a bit awkward but manageable.I've expanded my design to include an nRF9160, a 130-odd pin thing that's 10mm x 16mm. It seemed like a good idea at the time, but now they've arrived and I can see it in person I'm a bit overwhelmed. It's an LGA/QFN style package, so the pins/pads are all underneath and I expect they'll be difficult to align manually. It is obviously intended for automatic pick & place... but I don't have one.Has anyone ever successfully placed something like this? Any suggestions or tips?If the answer is that it's impossible and the only thing I can do is outsource the assembly then fine, but I'd rather not go that route.Any thoughts?Thanks!

author: Smokey
author_rank: Super Contributor
timestamp: October 17, 2023, 08:24:11 am
content: In my opinion, the hard part is getting the solder paste on right. If you get the paste stencil and squeegee technique down and get the right amount of paste, the part will "self center" as long as it's close when you place it.When I've botched the paste stencil and didn't want to wait for the new one, I've had some success "tinning" the pads with some solder and lots of flux. Try to get them really even. Then toss the IC on and reflow. Push down on the center once it's reflowing to make sure all the pads make contact.

author: Psi
author_rank: Super Contributor
timestamp: October 17, 2023, 08:31:46 am
content: Quote from: Smokey on October 17, 2023, 08:24:11 am+1 for this. For 1-off you can tin pads by dragging iron over them with lots of flux until you can get an even coating on all pads.Clean off the flux and then apply new flux, align chip, heat it up to reflow temps, and tap it down when ready so all pads join to pcb.

author: zzattack
author_rank: Regular Contributor
timestamp: October 17, 2023, 08:55:49 am
content: Package looks similar to a wifi AX210 that I swapped on a steam deck a few days ago. Pre-tin the pads on the PCB with a new even layer, same for the ground plane(s). Ensure the blobs are not too big, as when you push down on the chip excess solder is pushed outward and would cause bridges.Alignment is tough, but you should be able to see the smd pads extend (very minimally) on all 4 sides of the chip. With the chip aligned, hold it down with tweezers and heat one area briefly with hot air for ~10s. Then you can let go of the tweezers and reflow the entire chip. Small nudge test after reflow temp is reached will provide you the confidence of a good mount

author: loki42
author_rank: Frequent Contributor
timestamp: October 18, 2023, 07:37:33 am
content: Have you got a suction pen? Like the aquarium pump ones? They make placing stuff like this a lot easier I've found. QFN mostly self align pretty well though

author: Psi
author_rank: Super Contributor
timestamp: October 18, 2023, 08:05:35 am
content: ball of blutack on the tip of a small screwdriver works well too.Play around with the amount/surface area to get the holding for you need.

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: October 18, 2023, 11:19:48 am
content: Placed similar parts like the nRF9160. Use a tweezer and hold it at one of the corner of the module. Drop the perpendicular corner on the board aligned to the corner on the PCB, then slowly lower down the module before dropping off the last corner your tweezer is holding onto, avoid touching the paste at the last step. Just make sure your paste printing is perfect, the rest will be fine even with a bit of nudging for alignment correction. I do thousands of these because a lot of these modules comes in 44 mm tape with very large pitch which pneumatic CL feeder doesn't support. Need to get the electronic version to support the higher tape pitch which is $$$.

author: asmi
author_rank: Super Contributor
timestamp: October 18, 2023, 06:52:35 pm
content: It's the same as with BGAs - place alignment markers on the silkscreen such that they would be visible even if a package is at the upper end of size tolerance. If you use any magnification during placement, be mindful of perspective distortion (it can make properly aligned device seem unaligned towards the edges of a field of view) and check alignment at all four corners.

author: Smokey
author_rank: Super Contributor
timestamp: October 18, 2023, 07:36:38 pm
content: Quote from: asmi on October 18, 2023, 06:52:35 pmYa, this is a good idea for all parts without external pins. Make sure the corners are visible on the silk, and don't forget the pin1 marker visible too!

author: SMTech
author_rank: Frequent Contributor
timestamp: October 18, 2023, 08:22:16 pm
content: Or cheat, accept this is potentially quite difficult, skip the stress and buy this kind of module as a 3rd party assembled accessory you can plug into your design.Not sure why 48X doesn't simply implement the 48mm tape as a tray, it's what I would do although I believe the machine I use is much bigger so has plenty of space for doing this (intentionally).I would contend that as is frequently the case "self alignment" is overstated, things this big tend to stay exactly where you put them, they really don't like being smeared around a bit and the castellated versions are particularly prone to the perspective/parallax error issues when viewing them. Slight errors in paste alignment add to this error by playing tricks with your vision.Relying on silkscreen markers has its risks, silkscreen alignment is not always all that accurate - simply because its not intended to be used that way.

author: asmi
author_rank: Super Contributor
timestamp: October 18, 2023, 10:20:09 pm
content: Quote from: SMTech on October 18, 2023, 08:22:16 pmIt's been a while since I've seen silkscreen misaligned so much that it would be a problem for alignment. Almost all of my recent PCBs contain BGAs, and I'm yet to see any soldering issue with them.For super-small pitch BGAs you can add alignment marks in the copper to be absolutely sure, but they will occupy routing space in a place where it's almost always at premium. Or if you are that paranoid, include two angle brackets - one such that it should be visible, and another one inside it which should NOT be visible once package is properly aligned. I personally never felt like I needed to go that far, but it should make alignment easier for those who are just beginning to work with those packages.

author: Smokey
author_rank: Super Contributor
timestamp: October 19, 2023, 12:41:00 am
content: Quote from: asmi on October 18, 2023, 10:20:09 pmI've never seen silk alignment so bad this would be a problem. You would have silk all over pads if that happened....But to solve a problem that probably doesn't exist, you could route the signal traces around the corners of the IC such that they followed the outline of the package and use that as alignment. At least in the corners.

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: October 19, 2023, 05:36:33 am
content: Quote from: SMTech on October 18, 2023, 08:22:16 pmForgot to mention I ran out of head as well, hence placing it manually. Will try using larger nozzle that could hold those large module and at the same time smaller IC like TQFP and QFN. I'm so used to manual placing as before I had a machine, I done really a lot by hand. So sometimes the mentality is a bit hard to change even after having a machine.On the silkscreen, I always draw the silkscreen to be just outside of the body of the part. I don't see that many silkscreen that is so off position nowadays to the point I can't use them for positioning reference. But I do totally agree that such big mass part has no tendencies to realign itself during reflow. So, if you screw up during placement, you can kiss the chip goodbye. Removing them is also a tough job.

author: jpanhalt
author_rank: Super Contributor
timestamp: October 19, 2023, 08:59:51 am
content: I haven't seen anyone mention the order of build when placing by hand. Maybe I do it wrong or am repeating the obvious.With TH components, I learned to build from small to big. That usually translated from passives to others and finally the MCU (etc.). With solder paste, I found it usually easier to get good placement by doing the big stuff (IC's) first, then mess with the passives.Of course, there are exceptions, like parts that I intend to add later (e.g., switches) and parts (e.g., connectors) near the perimeter.John

author: FR4
author_rank: Newbie
timestamp: October 21, 2023, 03:48:40 pm
content: Many thanks to everyone here for the responses.Today I bit the bullet and gave it a go. I didn't have a silkscreen border, however the PCB pads are visible around the board even though the IC pins aren't. I was therefore able to try to keep the bits of pad that I could see equal all the way round. As the solder melted, I saw the IC move very slightly, hopefully aligning itself rather than mis-aligning. It probably wasn't the best paste job I've ever done (but too little rather than not enough - at least every pad had a bit on), it's definitely stuck on.Not a success yet though; I can't program it. It turns out that the nRF9160 requires a fancypants programmer that has "ARMv8-M support" - my cheapy one fails, hopefully it's the programmer rather than my soldering. I've got a better one on order to try it out. On the plus side, the programmer debug output said "Detected device family NRF91_FAMILY" before erroring, so it seem like a good start?!?Thank you all!