BTC/QFN Assembly Reality - Page 1
Forum section: electronics / general

author: forrestc
author_rank: Unknown
timestamp: February 28, 2019, 06:37:50 pm
content: I've found a part which is perfect for a revision to an existing product. Except that it's in a 10mm square QFN package. The good news is that the pitch of the QFN is only 0.8mm, which is coarser than the 0.5mm pitch TQFP's we assemble every day. The bad news is that there are 25 individual pads in the middle of this (1.5mm pitch, 0.6 diameter) - although most of them are going to be NC in this application.My assembly staff has had standing orders to shoot me if I ever utter the words QFN, BTC or BGA in relation to having us assemble a board in house - since I've obviously lost my mind. I'd like to figure out if I was being too harsh and can retract those orders.I have always been of the opinion that one shouldn't assemble QFN's without an XRAY. But recently I've heard several people say that QFN's are easier to deal with than a fine pitch TQFP and that inspection is largely unnecessary as long as you make sure that the paste is deposited reasonably correctly.So I guess I'd like it if those of who deal with similar QFN's with center pads could let me know how bad they really are to assemble. Information about dealing with inspection and/or defects/rework/etc would be greatly helpful. I'm really hoping I'm just letting FUD get in the way here, but I also don't want to go down this path if this is going to be more problem than it's worth (I'd probably just get this product assembled outside since it is going to be fairly low component count and I'd run several hundred a year).

author: SMTech
author_rank: Frequent Contributor
timestamp: February 28, 2019, 09:27:09 pm
content: QFNs really are nicer to work with than fine pitch QFPs. If you are pasting the board and reflowing it, not attempting to do it with a soldering iron you shouldn't have any problems, you can line up .8mm pitch ones with tweezers by eye quite easily (or .5 for that matter), just by gently placing them in place and giving them a nudge and they also self correct quite nicely as unlike a QFN the legs cant drop between the solder deposits. And because they are just square things event the shonkiest vision or alignment system on a p&p should manage them quite well.Arguably if you do need to rework them they can be a little tricky (but still way easier than a 100pin QFN), if you remove them with hot air there is often enough solder around to put a new chip straight back down, depending on any vias that might be around slurping solder away. Or you could flood the area with flux and re-wet the pads with a soldering iron Louis Rossman style (sorry I think the mans a cowboy but what he does works for him so, meh). Lastly another trick is I think called "bumping" where you paste and reflow your replacement QFN on its own and then rework it to the board with your trusty hot air pen.

author: forrestc
author_rank: Unknown
timestamp: February 28, 2019, 10:35:24 pm
content: Quote from: SMTech on February 28, 2019, 09:27:09 pmConsidering we're placing the QFP's perfectly with the vision system on the P&P, I'm guessing that you're right in saying that placing these won't be a challenge. I'll have to determine whether to have the machine vision use the body outline or the pads, but I'm sure that placement using one of those will be fine.One thing that I've heard in the past is that these are much more sensitive to PCB finish irregularities so one needs to use something like ENIG to get a flat surface (as opposed to something like HASL), and possibly be very picky to ensure you don't have any PCB warpage. Do you by chance know how much of this is actually the case?

author: SMTech
author_rank: Frequent Contributor
timestamp: February 28, 2019, 11:22:13 pm
content: I prefer enig because its reliably flat but unless your HASL is shockingly lumpy you should be fine

author: asmi
author_rank: Super Contributor
timestamp: March 01, 2019, 12:42:12 am
content: I only ever use ENIG on my boards as it gives super-flat surface (another good option is OSP, but it has a limited shelf life). This is super-helpful for BGAs. And yes - I take BGA over QFN any time because with latter I'm still getting bridges every once in a while, but I'm yet to have a single problem with BGAs.

author: forrestc
author_rank: Unknown
timestamp: March 01, 2019, 01:57:25 am
content: I guess the other question is whether or not I should bother with figuring out some way to inspect them other than a brief look around the edges to make sure that there aren't any obvious bridges....Of course, these will always get tested functionally here, I just worry about missing defects equivalent to a unsoldered pin which is making physical contact but doesn't actually have any/enough solder on it.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 01, 2019, 05:33:20 pm
content: Quote from: asmi on March 01, 2019, 12:42:12 amSame here, and this is also an opinion I have heard from several assembly houses.QFNs are also problematic for the cleaning step after assembly if you require one, as any residues will easily get trapped under them and often impossible to clean off, so you'll just have to live with it.

author: jmelson
author_rank: Super Contributor
timestamp: March 01, 2019, 10:18:40 pm
content: Quote from: asmi on March 01, 2019, 12:42:12 amI used to have a lot of trouble with bridging on 0.5mm TQFPs, and reduced solder volumes until I was getting open connections. Then, I switched to the Loctite GC10, and the bridging issue went away. Some magic stuff in their chemistry solves that. Now, I'm getting great results even on the 0.5mm pitch parts. I have only had one (VERY BAD) experience with leadless partts, don't have any desire to go back to them any time soon.Jon

author: forrestc
author_rank: Unknown
timestamp: March 02, 2019, 12:20:56 am
content: Quote from: jmelson on March 01, 2019, 10:18:40 pmSomehow we ended up with a paste that works for us from indium (well 2 since we both stencil print and dot-dispense). However, we have tried various alternatives over the years and have discovered each of them was worse in some ways - the most recent one I remember created defects where 0805 and 1206 passives would not be connected on one end, apparently due to an insulating layer of flux residue between the termination and the board. So yeah, I totally get the paste makes a difference thing.

author: asmi
author_rank: Super Contributor
timestamp: March 02, 2019, 05:13:25 am
content: Quote from: jmelson on March 01, 2019, 10:18:40 pmI use the same paste (Chipquik's version of it). My point was that many people consider BGAs hard to work with, but my experience indicates that QFNs were more problematic than BGAs ever were. Hence I prefer latter if there is a choice. The only problem with BGAs is some of them have very fine pitch and thus require using advanced PCB technology and/or creative layout approaches. I currently don't know any pcb fab that can manufacture boards with microvias for a reasonable money, so I limit myself to pitch 0.65 mm and above, which I can do with fabs I've been using.

author: forrestc
author_rank: Unknown
timestamp: March 02, 2019, 05:51:13 am
content: Quote from: asmi on March 02, 2019, 05:13:25 amThis is a good point for me to ask this question I guess....I've been shying away from BGA's and QFN's specifically because not being able to look at these joints scare me. I've heard the same thing about BGA's being better than QFN's. And the same thing about QFN's and BGAs being easier than the 20 mil pitch TFQP's we have on about 25% of the boards we do nowadays. The fact that "coarse pitch" BGA's in particular seem to be a lot lower technology devices as far as pitch and spacing goes makes me think this is true.But then I go to some electronics assembly show and set in on a session about soldering defects and they parade all of the pictures of head in pillow defects and BTC voids and so on. And they talk about how X-ray inspection is critical, especially since there are so many things which can go wrong with these components if your process is just a wee bit off. And then usually someone mentions that the real solution is something like vapor phase reflow. Which of course we'd all like to afford (right next to the X-ray machine). I then leave swearing never to touch these.So, I guess the question is this:How many of you who are having luck with QFN's and BGA's inspect using X-RAY machine or other method? And how many of you use manual stencil printers? And how many of you use a higher end reflow oven?

author: mrpackethead
author_rank: Super Contributor
timestamp: March 02, 2019, 07:06:46 am
content: where ever possible i replace QFP's with QFN's because I get less issues with them.

author: KL27x
author_rank: Super Contributor
timestamp: March 02, 2019, 09:47:53 am
content: ^QFP are a pain to handle. That's the main reason I avoid them.Can't really help the OP, though. I use QFN, but I outsource the assembly or use manual hot air or iron. And I didn't know there was a package called QFN that had more than 1 center pad. 25, you say? Until I read your post, I thought that would have been considered BGA. Or did you mean you're considering a 24 pin QFN with 1 center pad? A 10mm x 10mm 24 pin QFN is fairly gigantic. I can't imagine it being too problematic. I've used more pins in a package as small as 4mm.From my limited perspective on it, I would guess the main problems with reflowing QFN is the quality of the paste and the tendency of the chip to move if the paste gasses/bubbles too fast. So if I had issues, I would be looking at my stencil technique, freshness and quality/brand of paste, how long the paste sits and evaporates/hardens before reflow, and the reflow profile/ramp. It might take some tweaking for each new board?I'm not sure why BGA is less twidgy. Are they glued in place before the reflow?Sorry, if this is off topic, but have you considered test points on your PCB for pogo pins? This is a lot of time and work, but it might be viable compared to the cost of using an X ray machine.

author: OwO
author_rank: Super Contributor
timestamp: March 02, 2019, 10:10:49 am
content: BGA is good for production (and maybe planned obsolescence if that's your thing) but sucky for prototyping. You can not visually check that all the balls are indeed soldered so that's one more variable you can't eliminate. If you do find a mistake in your design you can not run bodge wires to the pads, which means you have to respin the board at the first encountered bug. All this means prototyping takes many more rounds and cost more in both wasted boards/chips and engineering effort. I would absolutely not recommend it in place of QFN unless this is a mass volume product and the saved space justifies it.

author: forrestc
author_rank: Unknown
timestamp: March 02, 2019, 12:26:37 pm
content: Quote from: KL27x on March 02, 2019, 09:47:53 amThe problem with BTC's is that some of the terms are pretty fluid. The manufacture calls this a LGA apparently, but it's not a LGA in my book. This package is more like a combination between a 25 pin LGA and a 44 pin 10x10 QFN. (See attached). I called it a QFN because I have looked at bona fide QFN's who have split the center pad into multiple parts (pretty common in power semiconductors in QFN packages - see the Microchip MIC28512 as an example) and I didn't look that closely at the datasheet.

author: KL27x
author_rank: Super Contributor
timestamp: March 02, 2019, 07:05:39 pm
content: From my corner of the universe, that looks like BGA and QFN combined. LGA is the other name that eluded me. My sample size is tiny, but the LGA I have used have no sidepads, for one thing, and they can have multiple center pins and unique footprints. This MIC28512 is the first "QFN" I have seen with multiple center pads. Thanks for the picture. I suppose some things might be called different things by different manufacturers. BGA = ball grid array. QFN = Quad Flat ... No external leads? I kinda thought of LGA as "other," a catchall. No clue what it stands for.For someone to whom QFN is a four letter acronym, you sure picked an interesting part on which to dip your toes back in the water.

author: mrpackethead
author_rank: Super Contributor
timestamp: March 02, 2019, 07:47:32 pm
content: Quote from: KL27x on March 02, 2019, 09:47:53 amYes, the handling of them is a real issue. Slightly damaged pins become a big issue quickly.

author: jmelson
author_rank: Super Contributor
timestamp: March 02, 2019, 08:16:52 pm
content: Quote from: forrestc on March 02, 2019, 12:20:56 amI got some Indium paste last year and was not happy with it at all! If you applied it when still a little cool from the fridge, it looked fine for 10 minutes, then started to spread into a blob. Once it was fully up to room temperature, it would spread out on the PC board immediately. I got LOTS of big balls on the side of the passives, and a lot of bridges and opens on chips. Totally unhappy with it, I only used it for two board runs.The GC10 is so much better. The only stuff in the same league is Warton, but there is no reseller in the US, and importing it myself is way too expensive.Jon

author: forrestc
author_rank: Unknown
timestamp: March 03, 2019, 07:42:06 am
content: Quote from: KL27x on March 02, 2019, 07:05:39 pmA BGA (Ball Grid Array) is an array of solder balls on the bottom, mostly in a row/column layout. An LGA (Land Grid Array) is basically a BGA without the balls - you provide your own paste. The older cousin to both of these is the PGA (Pin Grid Array) which basically has a pin on each land instead of a Solder Ball (BGA) or nothing (LGA).So my package would definitely be an LGA if it didn't have the QFN pads around the edge. Or a QFN if it didn't have the LGA portion. And yes, QFN is Quad Flat No-Leads. QFN's can have center pads just like TQFP's. And sometimes multiple ones like the MIC28512 and lots of other power semiconductors.BTC is the generic term for all of these no-lead packages - Bottom Terminated Component.And in somewhat good news it looks like I might be able to avoid bothering with the center LGA pads on this part at all since there are notes on the datasheet which say that it can be used as a drop-in replacement for the earlier part which apparently was QFN only. Which also makes sense if they were looking to enhance an earlier part while maintaining pin compatibility. I still haven't gotten in touch with application engineering at this manufacturer yet.

author: asmi
author_rank: Super Contributor
timestamp: March 06, 2019, 02:24:30 am
content: Quote from: forrestc on March 02, 2019, 05:51:13 amThis is a simple numbers game. Say, there is a 1/10000 chance of BGA soldering defect. That means if you do a 100 boards, chances that one of them fails is 1/100, or 1% - which isn't very likely. Assembly houses though assemble hundreds of thousands boards with BGAs, so even if their process is much better - say they have 1/100000 failure rate, that still pretty much guarantees that they will be getting those failures quite often.Quote from: forrestc on March 02, 2019, 05:51:13 amI use manual stencil printer (with framed stencils), and cheap ass T-962 reflow oven with some modifications (including some extra components and community firmware, as stock one just sucks). Tried soldering BGAs with leaded paste, room temp-stable lead-free and no paste at all (just a ton of flux - it was a small test board with a single BGA that I specifically designed to test and tune reflow process) - all three methods worked like charm, with all balls having good contact. I do profile pre-validation though for every type of board with a thermocouple attached to it to make sure the board reaches correct temperature and doesn't overheat - as different designs have different thermal mass.From my experience I found than QFNs are a lot more "picky" about having just right amount of paste. Or maybe - just thought about it - I have too long of soak phase so all flux burns off before actual soldering happens. Gotta do some more experiments. Also QFNs are generally harder to rework because they usually have bottom pad connected to big ground planes and so it can take A LOT of heat before the solder actually melts. While BGAs are quite trivial to replace using heat gun - at least smaller ones - say up to 100 balls or so, as I didn't try yet to replace any big BGAs simply because I don't have any specimens which are cheap enough that I wouldn't mind destroying few in case I do something wrong. Though new NXP's i.MX 6ULZ SoC might be a good guinea pig as it's sufficiently big (289 balls) while relatively cheap (5-6$ for qty 10)