[hackspacebristol] Soldering QFN packages

Ian Stratford ginjaian at gmail.com
Fri Jul 11 14:25:45 UTC 2014


Excellent! Added to the wiki page: bristol.hackspace.org.uk/wiki/doku.php?id=reflow-oven

Ian

On 11 Jul 2014, at 10:17, si at simonchambers.co.uk wrote:

> I’ve reflowed hundreds boards in my own [pizza/toast type] oven for a year or so - here’s a bit of a brain dump of what I’ve found/experienced:
>   - Even for one board, its quicker and easier than using hot air rework for a fresh board - it’s also a lot less violent than using hot air too. However hot air rework is essential if you need to remove & replace parts of course.
>   - Unless there are only a handful of components, I reflow my boards now. So much quicker and reliable.
>   - Get a stencil! You can use a syringe with tips, but unless if its for a one-off/small part count board with large pads, its hard to get the correct quantity on the board. For cheap stencils, I currently get them from www.elecrow.com - around $20 + shipping. Don’t bother with the enoooormous machine mounting stencils, just get the smaller cut out ones - cheaper shipping. Their boards are pretty good value too, however the drill hit sometimes can be a little off. I find them quicker at getting them made+shipped than Iteadstudio+seeedstudio.
>   - On the subject of PCBs, I thoroughly recommend www.hackvana.com if you want quality and cheap. He’s an Aussie bloke who lives in Shenzen who decided to offer quality PCBs. He has no online quoting system, but you can contact him directly by email and on #hackvana on Freenode. The quality rivals most western suppliers, and also if you have anything unusual he can cater for it - makes it much easier speaking to someone with English as their mother tongue.
>  - Again on PCBs, I rarely order from the UK/Europe now. You can get a PCB made in China, shipped (using DHL/FedEx) to your door in a week. Not only is it quicker than a lot of UK/European suppliers (unless you pay super money for superfast turn-around), you get 10 boards, for around 3rd of the price. Speaking to several people on the ground in Shenzen the equipment in the Chinese factories is the latest kit and top notch quality - better than a lot of European fabs. The cheap price is due to the highly competitive environment over there and that a lot of Fabs have had heavy investment in equipment (helped by the Government making it difficult for individuals take cash out of the country).
> 
>   - To mount the stencil, you need to have a right angle that you can push the board into and then you can tape the stencil along one edge, like a flap. You can get premade right angle boards, but I personally just use two scrap PCBs of the correct thickness and stick them together.
>   Another alternative to mounting the stencil, if you design it from the beginning, is to use a jig like this - http://www.hoektronics.com/2012/10/27/super-simple-smt-stencil8/ . You can buy a premade CNC’ed one from here: https://www.tindie.com/products/arachnidlabs/pcb-tooling-block-full-grid/ . This makes it a lot easier and quicker to setup. The guy who manufacturers/sells that CNC’ed on tindie is a active member of the London hackspace (Nick Johnson) - so I’m sure if Bristol Hackspace wanted to get one, he would happily offer a decent discount. He’s frequently on #hackvana and #london-hack-space on Freenode.
>   - When reflowing, the key temperatures I find are 150C, >200C (>220C for PB-Free). 150C allows the board and components to pre-heat up. Not hot enough to do damage, but hot enough to allow everything to reach the same temperature. You want to hang around there for 40-60 seconds. Then you want to heat it up fairly rapidly. When above >200C (>220C) the solder will start ‘reflowing’. You want to hit the peak temperature, and then shut off. Now you need to cool down fairly rapidly to prevent damage to the parts (not too quickly though). Personally I find opening the door on my oven is sufficient. Keeping it closed takes too long to cool down.
>   - I did make a controller with proper reflow profiles, etc, etc. It now just gets it to 100C (preheat the oven), then 150C and hold it for a max of 30-40seconds and finally heats up to 220C/240C (PB/PB-Free). Once it hits peak temperature, it buzzes to open the door.
>   - Don’t put the boards on a wire rack. Put it on a sheet of metal or similar. Note, make sure that the metal won’t bow as its heating up! Otherwise your carefully placed components will become scattered all around the oven, or even worse, become dislodged and reflow in the wrong place.
>   - Part placement (for the passives especially) is fairly uncritical, when the solder heats up, they’ll flow/move into the correct place.
>   - As Tom said, until you’ve done the process once or twice, don’t expect perfect results - especially if you’re doing QFNs. It takes a little bit of knack to get it right (but not too much!).
>   - Certainly early on, put a spare/scrap/similar board in the oven as well with a thermocouple attached to one of the pads/holes. This way you can measure the actual temperature of what the board is - rather than the ambient temperature in the oven. Otherwise you may find that the oven doesn’t get to sufficient temperature to properly reflow.
>   - Kapton tape is your friend, get some! You can get genuine 3M stuff but its hideously expensive. The cheap Polyamide tape is just as good for this type of work.
>   - EEVBlog/Dave Jones has a couple of good videos to get you started here - https://www.youtube.com/watch?v=FNNRoXZom30 / https://www.youtube.com/watch?v=JA-vi2iQ5vAWorth a watch.
> 
> Sorry for the length but I hope this helps!
> 
> Cheers,
> Si.
>  
> 
> 
> From: Tom Gardner
> Sent: ‎Friday‎, ‎11‎ ‎July‎ ‎2014 ‎10‎:‎03‎ ‎AM
> To: Bristol UK Hackspace
> 
> Arthur used the oven last night for the first time since modification. Before doing the "real" board, it might be worth trying a couple of sacrificial boards in order to assure yourself that you've got everything (temp, solder, flux, phase of moon) set up correctly.
> 
> I haven't done any SMD soldering, but I will have to in the near future. The links below may, or may not, be helpful - I'd be grateful for any experience/comments.
> 
> http://product.tdk.com//mlcc/en/applicationnote/pdf/30_rework_methods_for_surface_mount_mlccs.pdf
> http://store.curiousinventor.com/guides/Surface_Mount_Soldering/Tools/
> http://articulationllc.home.comcast.net/~articulationllc/sm0402.htm
> http://www.avrfreaks.net/modules/FreaksArticles/files/15/Low%20Cost%20SMD%20Soldering%20Guide.pdf
> http://www.dlharmon.com/solder/smd.html
> 
> I'm also going to be investigating using my slow cooker as a heat bath, since it will be more convenient for me to apply paste, then components, and then heat at home.
> 
> (No I won't be putting Pb solder in the cooking vessel! My slow cooker has a solid aluminium exterior that gets hot.
> 
> 
> 
> On 11 July 2014 09:43, Jolyon Jenkins <jolyonjenkins at googlemail.com> wrote:
> Hello
> I'm working on a project that will need me to solder some tiny QFN chips. I've never done this before. I see that the hackspace has a solder reflow oven but is there someone who could hold my hand on my first attempt? I guess I will need some stencils, unless the hackspace has them?
> 
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