How to solder component with hidden pads

[giraffe81]

I am looking at designing a PCB to take a Bosch BME280 sensor, however I am confused how I would solder it as the contacts are underneath the sensor. Details of the sensor here:

http://www.mouser.co.uk/ProductDetail/Bosch-Sensortec/BME280/?qs=2OnyuXx6vpj2fK9HX7qb3g%3d%3d

Thanks

[Rerouter]

hot air rework station,

[giraffe81]

They don't seem to come cheap. This is fast becoming an expensive hobby 

[AlxDroidDev]

They don't seem to come cheap. This is fast becoming an expensive hobby 

You should try photography as a hobby... then you'll see what an expensive hobby looks like!

A cheap alternative (with pros and cons) to a hot air rework station is a Dremel Versatip (aka Dremel 2000).

[giraffe81]

You should try photography as a hobby... then you'll see what an expensive hobby looks like!

Funny you should say that, I used to do quite a lot of photography and it was frustrating not being able to afford the lenses.

Anyway, thanks I will take a look at the Dremel. Although I see you can pick up a hot air rework station on ebay for about £30-£40. I am sure it's not the best but might be passable.

[tggzzz]

There are many ways of doing it; google and youtube are your friend. Skillet, toaster, drag soldering are keywords.

The main problem with that method is that if the PCB and component aren't at the same temperature, then thermal stresses will arise during cooling. For amateurs, it is preferable to use components with leads, even if they are "tucked under", or possibly components with contacts up their side.

[rgawron]

Hot-air stations don't have to be expensive, this one is decent - I have this model and I'm Ok with it (852D+ hot-air):

http://www.ebay.co.uk/itm/2in1-852D-SMD-Iron-Soldering-Rework-Station-Hot-Air-Iron-5Tips-ESD-PLCC-BGA-/141192509493?hash=item20dfbab835:g:xgcAAOxyLm9S~Wn9

You will also need a solder paste in a syringe, something like that:
http://www.ebay.co.uk/itm/SMD-Solder-Paste-SN62-PB36-Ag2-8g-Syringe-For-phones-Xbox-computers-etc-/121275877627?hash=item1c3c9b08fb:g:cPsAAOSwwpdW9s1D
Most if not all of them have expiration time and need to be keep in cool temperature. Also you need some needle to apply it.

It will be much easier if you will chemically tin the PCB before soldering:
http://www.tme.eu/gb/details/cyn-chem-45/materials-for-pcb-production/ag-termopasty/preparat-do-cynowania-miedzi-agt-111/
IIRC it also have expiration date and after mixing with water can't be keep for a long time.

Tinning agents are relatively expensive, but they can be much cheaper made from separate ingredients also available online, recently I've found this mixture (thiourea + tartaric acid + tin(II) chloride) mixture and it seems to be really good (yes, my blog, but context related). Ayway if you're not into chemistry just buy te ready-to-use solution.

If you want to make more units, you may want to order a plastic sheet with cuttedof places to apply the paste, but I it's not mandatory.

[hayatepilot]

They don't seem to come cheap. This is fast becoming an expensive hobby 

Definition of a Hobby: "To use as much money as possible to make as little profit as possible." 

Dave did a video how to solder such chips:



Greetings

[nanofrog]

I will take a look at the Dremel. Although I see you can pick up a hot air rework station on ebay for about £30-£40. I am sure it's not the best but might be passable.

I'd skip the Dremel (mini butane torch) as there's no temperature control.

As per methods, see above. But since I have a hot air rework station, I'd opt for that first since it can get the heat under the component and is easy to do. Another good one that's easy & feasible for this particular package would be the electric skillet method. Either of these methods will require a good quality solder paste, such as Kester's Easy Paste 256 in 63/37 (available in a rework friendly size syringe from a 3rd party company here in the US, CML Supply).

If you'd like to find a high quality station that won't cost what the major brands do, I suggest taking a look at a Quick 861DA (the A at the end designates it's a 220V, UK plug model). BTW, if you search for Quick 861DS and Quick 861DW, there's a good bit of information that comes up, including internal photos.

[giraffe81]

Thanks for all the info, has given me something to think about. Will probably stick to the breakout board containing the sensor for now. My soldering isn't that great so probably need to improve that before getting out the solder paste and toaster. I can see the look on my wife's face already 

[ade]

Yeah many hobbyists prefer working with breakout modules.  And at low quantities, often buying complete modules is cheaper than buying individual chips -- as is the case here.  (BME280 modules are around $5 on eBay shipped -- cheaper than the chip alone from Mouser).

Personally I'm the other way around; I prefer to work with SMD and I avoid through-hole components.

[Mephitus]

As another option, you can get/use a toaster oven if you utilize an oven controller to moderate the heating profile.
Here is what I use: https://www.eevblog.com/forum/projects/oven-reflow-controller-(rocket-scream)/msg892874/

[Jeroen3]

The thing with hot-air is that if you apply the right flux, you just have to hold the air gun for a while.
-First pre-tin the pads. Don't use paste, it will blow solder balls everywhere.
-Apply some flux.
-Place the component.
-Heat it up.
And the surface tension of the solder will pull the part in place.

You should see that on the videos.

[alanb]

If its a one off job you could consider mounting the chip upside down on the board, fix it in place with dab of superglue then solder fine wires between the component pads and the PCB.

--- I have just noticed that the chip in question is a humidity sensor, if you used the above method you will need to mount it over a hole in the PCB to allow access to the hole in the sensor.

[macboy]

An alternative might be to dead-bug it. Flip it over, and solder some wires (perfect example is leftover leads from through hole resistors or diodes). Then you can bend the leads into a DIP-8 configuration and solder to board. For more mechanical durability, insert those leads into a machined pin DIP-8 socket (don't bother using a leaf spring socket), then optionally solder them in place. Then use that as a DIP-8 component on your breadboard or wherever. I've done this with those tiny little oscillators from cell phones.. they generally have excellent accuracy and stability.

[LA7SJA]

How can the air hole on top of the chip work upside down? I think it would live up to the name "Dead Bug".

Johan-Fredrik

[dadler]

So, for QFN and similar packages like this, I typically just make the pads extend outside the footprint of the package (when laying out the board). Then you can directly heat the pad with a soldering iron--with enough flux and pre-tinning the pads, solder will flow under the package.

Or use hot air.