How to solder DFN/QFN/HLGA/etc - Page 1
Forum section: electronics / general

author: hpmaxim
author_rank: Regular Contributor
timestamp: September 19, 2021, 09:19:20 pm
content: I bought some PCBs from JLCPCB with ENIG finish and I'm trying to solder some of these PITA packages with tiny pads that are around the perimeter but under the package. I got a stainless steel stencil from JLC. I saw a technique on Youtube in which they mounted spare PCBs (with spray glue) so that the PCB to be soldered is held in by friction and the stencil is held flat over it. I then taped the stencil in place, and used Chip Quik Bismuth low-temp lead free paste. I did the best I could to place all the parts on the board with tweezers and then stuck the board in my Controleo 3 oven (I tried with the leaded profile first, and then tried again with the lead-free profile). All the larger surface mount parts soldered fine, but only 1 of the 4 tiny pitch parts worked after being soldered down. This is the second time I tried, I had similar problems the first time.I was never really super happy with how the solder paste laid down. In some cases, the paste seemed to be thicker/higher than it should be. Also, I found that slits in the stencil were so narrow it was sometimes hard to get the paste through it. In fact, before using it a second time, I found many of the smaller slits were plugged up, I tried cleaning it with soap and water and with scraping it, and it wouldn't come off. I eventually stuck it in my ultrasonic cleaner and that got the stencil clean.For those unfamiliar with Chip Quik, it actually is lower temperature than most leaded solder, so that's why I used the leaded profile.Anyone got any suggestions?

author: bson
author_rank: Unknown
timestamp: September 20, 2021, 12:32:03 am
content: I put a little leaded paste on the pads with a toothpick under a 5X microscope. Then place the parts. Hot air (Quick 861DW), about 305°C, very low flow rate, and they solder just fine. I don't bother with stencils, the toothpick method is quick and easy - although it takes a little practice. The paste is MG Chemicals Sn63/Pb37.That said, if JLCPCB had the parts I need I'd probably just order boards assembled.

author: hpmaxim
author_rank: Regular Contributor
timestamp: September 20, 2021, 01:33:30 am
content: I've thought about using JLC's assembly service, but they have a very limited selection and their reel change fees really run up the price on small quantity jobs. I'm going to try to go through PCBWay which will allow you to give them a parts kit.Thanks for the tip about the solder and toothpick. I tried something similar with the tip of tweezers, but found it challenging to lay it down without getting too much on and still getting good coverage. I then soldered it with my YouYue 858D+, but I'm pretty sure I ended up bridging pads. If a toothpick and different solder paste help I'm all for it though.

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: September 20, 2021, 02:03:10 am
content: Show picture of the applied paste without components?

author: Cerebus
author_rank: Super Contributor
timestamp: September 20, 2021, 02:07:17 am
content: I had a little experiment recently. I ordered two sets of identical boards from JLCPCB specifically to have a play with some tiny QFN parts. One set with ENIG, one set with standard leaded HASL. The packages were QFN16 logic gates (74HC595) in 2.5 x 3.5 mm packages. Used a stencil, hot air and three different 63/37 solder pastes, one with some very old paste with a little silver in, one with fresh chipquik straight 63/37 and one fresh mechanic 63/37 - all on ENIG. All went well first time. Of all the solder pastes the cheapo mechanic produced the best finish. I also tried mechanic on the HASL version of the board. Again, no problems, worked well first time.The thing that sticks out as significantly different is that you're using an odd-ball low temperature paste. From what I've heard the thing that will get you every time with QFN is "too much paste" especially on centre pads. It has never given me problems, but I've heeded others advice to use fairly aggressive paste aperture reduction for body pads and a moderate shrink on the perimeter pads.Full details of my experiment copied from a previous post -As a reminder, this is the stupidly small package that I'm working with:I have stock of three different solder pastes that I've tried out on the same part and board. The first is some old Sn63Pb36Ag2 with type 818 flux that I've had sitting in the fridge for a couple of years; I've lost its pedigree but it's a respectable brand that someone was selling on ebay in economical size syringes that they'd filled themselves with something that normally only comes in giant pots (Multicore, Kesler, something of that ilk). The second is a syringe of fresh Mechanic brand XGZ40 (a common cheap Chinese variety sourced via ebay). The last is a virgin syringe of Chipquik SMD291AX bought from RS. The latter two are straight 63/37 alloys. All are type 3 mesh.The results:Old Sn63Pb36Ag2MechanicChipquikWorked fine, a tad drossy, best tackOK, less tacky than the firstLeast tackyAll those results are on boards with an ENiG finish. They all self-aligned equally well, any apparent differences are down to camera angle.So, my conclusion? The best feel, the easiest to use was the old solder but the results are a bit drossy - it's too old. The Mechanic and Chipquik produced comparable results but the Mechanic was a little bit easier to print and slightly tackier. The mechanic is also about £5 a syringe and the Chipquik £15. So I'm going to stick with the Mechanic for the time being and keep my eyes open for some quality high tack paste with some silver content at a reasonable price. All were good enough to get the job done.For the sake of experiment I also ordered some HASL(leaded) finish boards at the same time. All the advice I've ever received as "Use ENiG for QFN packages" so I wasn't too hopeful for the results. I was wrong, the HASL boards were as easy to place these tiny QFN packages on and get finished results as good as the ENiG boards. No pics because they're not really helpful, you can't distinguish the results from the ENiG board done with the same paste. Now, this is a one-off, but the takeaway is that fine pitchwillwork on HASL - your mileage may vary. Why this matters is that the ENiG finish option alone costs significantly more than the boards most of the time from people the likes of JLCPCB (£12.25 extra for ENiG on a £1.44 100x100mm board).The complete 25 x 50mm test board.This one was soldered with the old slightly tired paste that wastechnicallywell out of date.

author: DIYGUY Chris
author_rank: Contributor
timestamp: September 20, 2021, 08:53:38 pm
content: The closer you get your Stencil to your PCBs the superfine the solder paste layer you get, that's the rule.Dropping solder paste manually is not that much hard if you preheat the PCB first, once the solder paste touches a hot surface it get slightly molten but not totally molten so you still can place your component, that what I do for small components packages and it works fine

author: hpmaxim
author_rank: Regular Contributor
timestamp: September 21, 2021, 12:20:20 am
content: Hi Cerebus,Thanks for the detailed response. I was using ENIG, because I had been told by someone with much more PCB level experience than I had (I'm an IC guy) that ENIG worked a lot better than HASL for fine pitch components. But perhaps, you found out, that's not so true. I was using the "oddball" paste because it was recommended to me by the guy who made the reflow oven after being unsuccessful with a conventional lead-free paste.I think you probably hit the nail on the head though with your other comment. I suspect I may have had too much paste on the die paddle/EP. I found it very challenging to get paste onto the actual pads through the stencil. And as a result I kept working the paste over the stencil over and over. It occurs to me, that if you get too much paste on the pads it can flow out from under the package, but if you get it too much under the die paddle and it flows out it's likely to short. Perhaps, take only one pass across it, and then use a q-tip to spread the paste over the entire area and if it's just the right, fairly uniform, thickness maybe stuff will just work out. I think I need some practice packages and then just make a PCB and try different techniques.

author: hpmaxim
author_rank: Regular Contributor
timestamp: September 21, 2021, 12:32:51 am
content: Actually, I may be an idiot. There was no die paddle on most of the parts that didn't work... It is still possible I put on too much paste.The LGA parts are U1, U2, U3, and U10. U11 is a DFN. U1 definitely worked. U2, U3, and U10 likely did not work. U11 probably worked.U1, U2, U3, and U10 were all on different power supplies but were on a shared I2C bus. I was able to communicate with U1, but not the others, Power consumption was normal (about 9mA).... So I suspect U2, U3, and U10 had one or more opens (unlikely a short between power and ground, or between either and I2C clock or data). When U11 was installed sometimes power consumption was normal, other times it was about 45mA high (U11 was an LDO regulator that dropped voltage from 5 to 3.3V) so I don't know if U11 was drawing the power or if it was a load on U11 that was drawing the power. However, a similar (same schematic, different layout) board that was manufactured by PCBWAY only draws about 10mA when powered up (and U1, U2, U3, and U10 are each working).

author: jmelson
author_rank: Super Contributor
timestamp: September 21, 2021, 12:36:52 am
content: You need to use a thin stencil, and also reduce the aperture size to maybe 60% of the pad area to get these leadless parts to solder properly. If you have a "brick" of solder paste when you plant the component, the paste squeezes out sideways and causes shorts.Jon

author: jmelson
author_rank: Super Contributor
timestamp: September 21, 2021, 12:38:28 am
content: Quote from: hpmaxim on September 21, 2021, 12:32:51 amOH, YES! WAYYY too much paste for the fine pitch parts.Jon

author: Cerebus
author_rank: Super Contributor
timestamp: September 21, 2021, 12:49:48 am
content: Quote from: hpmaxim on September 21, 2021, 12:20:20 amThat's what I had as received wisdom too. The issue was always cited as 'coplanarity' or flatness in simpler English. Now, JLC seem to have done a very good job of producing a good flat finish on HASL boards I've had from them, which persuaded me that it would probably work OK. The minor cost of trying it just as an experiment was negligible. I've seen much bumpier HASL so it may be a very risky thing to do if your PCB supplier isn't up to the mark. Also, long term solderability is better on ENIG compared to HASL. Who knows, perhaps if you took a blank HASL finished board off a shelf where it had been sitting for six months the results might not be so good.I'm just chancing it on hand assembled boards for hobby prototypes. If I was doing it for pukka production purposes I'd probably talk to the production engineers and listen to them. I'm deliberately pushing the edges of the envelope on experimenting with what most people regard as 'tricky' packages because I don't want to be limited in choices of devices - some interesting things I might use are only available in 'awkward' packages. Also there's the simple pleasure of trying to produce the most compact and neatest looking board possible - once one has of course taken care of any technical layout requirements first.

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: September 21, 2021, 01:55:30 am
content: Quote from: jmelson on September 21, 2021, 12:36:52 amThe guys at JLCPCB/PCBWAY/InsertYourManufacturerHere will swap your stencil thickness based on your components pitch. If they felt that your selected thickness is too thick for the design, they will use a thinner one.Quote from: hpmaxim on September 21, 2021, 12:32:51 amYour paste are smeared beyond your pads dimension. This due to 2 possible reason:1. Your paste are not stirred well enough before use and mostly the liquid part ended up on the pads.2. Your are pushing paste onto the stencil by having a too small angle (basically close to lying flat) while holding the squeegee. It should be around 45-60 degree and run the squeegee across the stencil ONCE. I don't blame you as there's truckloads of wrong videos on YT and IG doing this and most people followed all that.Attached is how the paste should look like on finer pitch components. If it doesn't look like that, don't waste your time and components to reflow. Just wipe clean and start all over again.

author: JLCPCB Official
author_rank: Regular Contributor
timestamp: September 21, 2021, 02:38:58 am
content: The Stencil thickness could be adjusted for some specific areas as it shows in the attached image and it could go up to 0.06mm as minimum thickness for step area.If I consider the description from your post about the assembly tip that you have learned from YouTube then I can say that it is the appropriate one, just make sure that you keep a flat Stencil and stick it to your board without keeping any space because the curved Stencil could drop more solder paste which makes the paste layer thicker than what you expect.Maybe if you share some images then we can give you more assistance and tips.

author: Cerebus
author_rank: Super Contributor
timestamp: September 21, 2021, 01:51:46 pm
content: You shouldn't need to go to a stepped stencil for a board like the OPs. Stepped stencils are for where you have a mix of parts on a board such that some need lots of paste and some need very little - say mixing 0.4mm pitch CSPs with DPAKs or some similar "very little and very large" combination where in an ideal world you'd want say a 0.06mm stencil for the CSP parts and a 0.15mm for the monster parts. Stepped stencils tend to be rather costly too.As far as the OPs smeared out paste deposition goes: It looks very much like too little pressure was applied to the stencil itself (as opposed to pressure applied to the squeegee) leaving a gap between stencil and board that allowed paste to squeeze out. I usually tape the stencil firmly on one edge only to form a hinge and keep that edge of the stencil in good contact level with the board and then bridge my fingers from one hand pressing down across the working area to push the stencil hard against the board and work the stencil within the area I'm holding down. I usually press quite hard with the squeegee (OK, with the old credit card) working the paste into the stencil until I can see that all the areas are properly filled and then make a single pass over everything with the squeegee at 45, perhaps 60, degrees to level everything off - pressing hard enough to sweep the surface of the stencil clear of paste.I'd suggest perhaps a deliberate practice session putting some paste on boards, inspecting it, wiping it off and repeating until you get a result just like 48X24X48X's image (repeated below). One thing that is often not emphasised enough is the importance of keeping your stencil clean. Before I start a session I always clean both side of the stencil with some isopropanol, and while I'm printing I'll regularly clean the back of the stencil either every 5 prints or so, or the second that I stop getting a clean, well defined print.That picture from 48X24X48X is exactly how your paste should look. Note the crisp edges, that sense of having a little vertical edge to each deposition of solder paste.

author: bson
author_rank: Unknown
timestamp: September 23, 2021, 08:22:31 pm
content: Interesting that HASL worked well for you. The few times I've tried it I haven't been able to get the tinning to melt, and end up soldering onto the tin rather the underlying copper pad. This is JLCPCB leaded HASL even. No matter how I try, I end up destroying something before the tinning melts. Either the board is singed, the part overheated, or a pad lifts altogether. Or the QFN package separates from the contact. No timing or temperature or flux flooding helps. As a result I only do ENIG from JLCPCB for SMT. Ever.

author: wraper
author_rank: Unknown
timestamp: September 23, 2021, 08:51:56 pm
content: Do only a single pass with a decent metal squeegee when applying solder paste. Multiple passes often cause solder paste seeping sideways like on the photo.

author: hpmaxim
author_rank: Regular Contributor
timestamp: September 26, 2021, 04:51:52 am
content: Let me respond to a few things here:1) I agree that the stencil may not have been completely flat against the PCB. I did my best to get it that way, I had electrical tape on all four sides of the stencil trying to pull it taught. It still didn't seem perfect and I wasn't super happy with it. I tried to describe what I did in the first post. Again, I glued four scrap bare PCBs to a flat tile, and then put the actual PCB in between the scraps so it was held on by friction, aligned the the stencil to the actual PCB an then taped each edge to the scrap PCBs, this should have resulted in the stencil being pretty flat against the actual PCB.2) I did run the squeegee (credit card) over it repeatedly. The HLGA-10 part has VERY VERY narrow pad slits and as a result, the first couple times I tried to squeegee it, I don't think any paste transferred to the part. It's always possible that the solder paste wasn't ideal, perhaps it was too thick...

author: m98
author_rank: Frequent Contributor
timestamp: September 26, 2021, 12:55:26 pm
content: Only tape one side of the stencil. you want to lift the stencil up in one quick motion, so that you don't smear the paste in the process. Also, while plastic cards kind-of work as a squeegee, I never had much success with that. Get a metal one with a nice sharp edge with which you can really press down on the stencil, and use excess paste, so you don't run dry. Before printing, I also wipe down the boards with IPA to get rid of any fingerprint-grease.

author: hpmaxim
author_rank: Regular Contributor
timestamp: September 28, 2021, 03:24:52 am
content: Quote from: m98 on September 26, 2021, 12:55:26 pmIf only one side were taped down then it would likely be moving around and would not be taught. Ideally it should be removed vertically upward, which isn't going to happen if there is ANY tape on it.Looking on Digikey, I only saw plastic or rubber squeegees. There was a Hakko spatula for $10, or I found this:https://www.amazon.com/dp/B0093HWGOOHowever, I can imagine that stainless or stainless might be a bit rough on the stencil.

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: September 28, 2021, 04:12:53 am
content: Quote from: hpmaxim on September 28, 2021, 03:24:52 amShould only be 1 side tape down. Your other hand should press it down on the opposite side that is not being taped down. Stainless squeegee is easily available on AliExpres for $5 for 10cm version. Once you use them, you won't want to go back to your plastic card. I have been doing stencilling manually with and without stencil frame for more than 30k boards, it's all about practicing and concentration.

author: hpmaxim
author_rank: Regular Contributor
timestamp: September 28, 2021, 06:36:35 am
content: Can you show me an example on AliExpress? It seems like all I'm seeing show rubber blades.

author: xlnx
author_rank: Contributor
timestamp: September 28, 2021, 07:51:30 am
content: Quote from: hpmaxim on September 28, 2021, 06:36:35 amhttps://www.aliexpress.com/item/4000405954741.htmlI would also suggest - as 48X24X48X pointed out earlier - that you stir the solder paste properly. In storage, the flux and tin will separate. Before I learned that, I had exactly the same results as you with paste just floating away from the pads. Stirring will restore the proper concistency of the solder paste.

author: hpmaxim
author_rank: Regular Contributor
timestamp: September 28, 2021, 02:33:08 pm
content: Ahh... I was searching: "stainless squeegee" when I search "SMT squeegee" or "stencil squeegee" results are much better. Is there a recommended size?

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: September 28, 2021, 02:39:26 pm
content: Depends on your PCB size. Should have extra on both side of PCB. Buy a few for different length PCB. I have 10, 15 and 20 cm.

author: SpacedCowboy
author_rank: Frequent Contributor
timestamp: September 28, 2021, 09:17:58 pm
content: Quote from: hpmaxim on September 20, 2021, 01:33:30 amFWIW, I've found that manufacture/assembly from nextpcb.com is cheaper than pcbway, pretty much all the timeDon't work there, yadda yadda...

author: hpmaxim
author_rank: Regular Contributor
timestamp: October 03, 2021, 12:55:27 am
content: Quote from: SpacedCowboy on September 28, 2021, 09:17:58 pmWhen I priced it out, nextpcb seemed way more expensive.

author: SpacedCowboy
author_rank: Frequent Contributor
timestamp: October 03, 2021, 06:53:54 pm
content: > When I priced it out, nextpcb seemed way more expensiveTo be clear, I was talking about nextpcb being cheaper than pcbway (and I've used both) but maybe it's more to do with the type of boards I'm doing. Typically I've got 6 layers and big BGAs, including assembly. Generally I find nextpcb to be $100->$200 cheaper when doing the prototype run of 5.