SMD question

[SwedishWings]

Hi fellow diy:ers,

I have been doing double sided PCB's for ages, but only through-hole designs.

I'm now going to make a prototype series of a 20mm X 30mm double sided SMD board with quite high density (SSOP-28, QFN-24/4x4mm etc), and would like to ask for advice;

1) Is it easier to use a hot air soldering station or using an oven to re-flow?
2) I previously used a 600 dpi laser printer to do the films. I sandwich the films and the board in my UV box. Will this method work for SMD as well? Or do i need better films than a laser printer can do?
3) What typical issues does guys like me, going from through hole to SMD designs, typically screw up on?

Any other hints are most welcome!

Thanks,
Mike

 

[KeepItSimpleStupid]

1) I know of three methods: Hot air, the frying pan and a ramp/soak controller converted toaster oven. The toaster over would be very similar to commercial methods.

Generally, you will require a stencil to lay down the solder paste. That may be the harder part.

2) You really need to check your resolution. I know these are expensive: https://www.thorlabs.com/NewGroupPage9.cfm?ObjectGroup_ID=4338 but they allow you to check resolution. I looked for "photolithography test targets". You can try making your own for the smallest packages you have. Put the pads down and attempt to draw parallel lines from them. There is a cheaper source on the new somewhere.

3) No clue. I did some thru-hole designs using Melles Griot mask aligner because we had one. I found some translucent Polyester paper lying around that worked wonderfully for creating a mask. I think there is a good source in the UK. I used a drop of water between the paper and the board to get intimate contact between the board and the paper.

Since my boards were spin coated, my method of aligning the sides wasn't the best. I drilled 2 holes in the board and the mask prior to spin coating and baking. Then I used cut off tacks to align the mask.

When cleaning the copper, I used a mixture of Vinegar and salt.

I don't know how important a solder mask will be. I suspect, that in some cases it may be essential.

 

[DirtyLude]

How many of these boards do you plan to make?

For simple one off prototypes, you don't need a stencil and it really doesn't matter what method you use. A hot air gun comes in handy as a desoldering tool though, so you might want to keep that in mind. I get a long fine with a syringe of solder paste. If you are doing a few of the same board, a stencil sure comes in handy, though.

I use a cheap HP laser and toner transfer and don't have any trouble doing QFN's. I'm assuming with photo you should be fine, but that's based on your actual process.

I would suggest just start doing it and see how it goes. Adjust your process from there.

 

[edeca]

Most people do not use hot air to actually solder a board. It is very useful for removing and reworking parts, but not as good for actually soldering. A decent iron with a fine tip is much better at this.

I detailed my process a few years back on my blog, it hasn't changed too much from then: **broken link removed**

Buying proper PCBs makes SMD much easier at fine pitch. There are all the issues of resolution, but you can easily do some SMD when you've got your DIY process improved. However manufactured PCBs should come with a soldermask which makes soldering much easier. Unless you apply way too much paste the soldermask will generally keep the solder to the correct places!

My current favourite hobby PCB manufacturer is Laen at https://oshpark.com/ - others are also available.

You don't need a stencil for low volume designs, but it can help if you want to scale up manufacture.

 

[kubeek]

Most people do not use hot air to actually solder a board. It is very useful for removing and reworking parts, but not as good for actually soldering. A decent iron with a fine tip is much better at this.

The problem lies in placing the part correctly, which is a problem when you have a tinned board and try to place a large device on the bulging tin.
When you don´t have a stencil and simply apply the paste from a syringe, the best method is to use hot air to melt the solder paste and, with a bit of percussive maintanence, let the part get perfectly centered by capilary forces of the molten solder. After that you usually get lots of bridging because of too much paste applied, so you wick the excess solder with standard soldering iron, then apply liquid flux and reheat the pins again with standard soldering iron just to be sure.

With this method I had no problem placing 144pin 0.5mm pitch LQFP devices.

 

[DirtyLude]

Most people do not use hot air to actually solder a board. It is very useful for removing and reworking parts, but not as good for actually soldering. A decent iron with a fine tip is much better at this.

For IC packages with leads I also don't bother with the reflowing them. It's easy enough to solder with an iron. With passives, it's easy to dab down solder on all the pads, drop the 0603's or 0802's and then hit it with hot air. Easier than tinning a pad and setting the component then soldering the other pad. The OP mentioned QFN's, though and unless you have a significant amount of pad sticking out, it's difficult to do a QFN with an iron.

... soldermask which makes soldering much easier. Unless you apply way too much paste the soldermask will generally keep the solder to the correct places!

I disagree on the usefulness of soldermask. I'm not certain why I don't have a problem with this and others seemingly do, but I have no more bridging with soldermask than without.

 

[SwedishWings]

I'm blown away... Thanks folks! My first post here only a few hours ago, and already a bunch of healthy information for a noob =)

2) You really need to check your resolution. I know these are expensive: but they allow you to check resolution. I looked for "photolithography test targets". You can try making your own for the smallest packages you have. Put the pads down and attempt to draw parallel lines from them. There is a cheaper source on the new somewhere.

This makes me worried. You mean that i need to get better films? I currently live in the Philippines, and this place pretty much sucks when it comes to do things like that =/

How many of these boards do you plan to make?

I use a cheap HP laser and toner transfer and don't have any trouble doing QFN's. I'm assuming with photo you should be fine, but that's based on your actual process.

I will make max 5 boards. When all is good, i send it to China or SG for manufacturing and assembly. The thing is this; I always made prototype boards with the same gerber files as i send for manufacturing. That way i know for sure that the final production gerber files will work when i get them back.

The problem lies in placing the part correctly, which is a problem when you have a tinned board and try to place a large device on the bulging tin.

You mean that it's advisable to tin the boards before place & re-flow? I have never tinned a board, usually apply some protection (spray) after completed assembly if need be. If necessary, what process for tinning do you suggest?

With passives, it's easy to dab down solder on all the pads, drop the 0603's or 0802's and then hit it with hot air. Easier than tinning a pad and setting the component then soldering the other pad.

I disagree on the usefulness of soldermask. I'm not certain why I don't have a problem with this and others seemingly do, but I have no more bridging with soldermask than without.

What minimum sizes do you recommend for resistors and similar?

I will have a very hard time to find someone doing paste masks here.

Many thanks to all of you helpful humans out there!

Regards,
Mike

 

[SwedishWings]

I use a cheap HP laser and toner transfer and don't have any trouble doing QFN's. I'm assuming with photo you should be fine, but that's based on your actual process.

Could you elaborate on the process? What films do you use? I typically printing straight on "over head" films from HP, it's been good so far, but perhaps i need to refine my process?

 

[edeca]

What minimum sizes do you recommend for resistors and similar?

With average soldering ability and good eyesight you can easily do 0805. With good soldering technique or a little practise 0603 is also quite reasonable. I've never gone smaller than this but check out any modern PC peripheral for how small some stuff gets!

I tend to use 0805 for most things and 0603 for common items like 0.1uF capacitors. You do need a decent iron with a fine tip. Temperature control also helps, personally I wouldn't now work without it but others probably manage just fine.

Anything package with leads generally isn't too difficult. This accounts for most ICs and transistor packages such as SOT-23. I missed the part in your post about QFN, packages with no pins and "hidden" pads can be more difficult and hot air will definitely help.

I made up the board below with illustrations of how small the pads are. Happy to share the gerbers if people would like to get one made up. I don't have any spares right now to send out unfortunately.

View attachment 68099

 

[tvtech]

Hi edaca

I have designed a LED head....and am going to try and assemble this ....by hand..for the time being. Here are the LED's I will be working with...

Please see PDF attachment. Is this possible???

Regards,
tvtech

 

[SwedishWings]

With average soldering ability and good eyesight you can easily do 0805. With good soldering technique or a little practise 0603 is also quite reasonable. I've never gone smaller than this but check out any modern PC peripheral for how small some stuff gets!

Thanks a lot for your input! The design calls for use of quite a few 0603's, so i guess my patience will be put to test I just ordered a new lamp with magnifier glass, guess it will come in handy!

 

[edeca]

Hi edaca

I have designed a LED head....and am going to try and assemble this ....by hand..for the time being. Here are the LED's I will be working with...

Please see PDF attachment. Is this possible???

Perhaps start a new thread and point me at it?

 

[DirtyLude]

Could you elaborate on the process? What films do you use? I typically printing straight on "over head" films from HP, it's been good so far, but perhaps i need to refine my process?

Just search for toner transfer. It's a process where the board outline is printed on paper and then ironed on the copper clad. If you are using photo transfer and are happy with it, I would stick with that. Toner transfer is much more finicky and can take a good amount of work to get right.

I took these pictures of the process for this forum a few years ago when I was testing out a different type of paper.
**broken link removed**

 

[jpanhalt]

If you decide to try toner transfer, and for those who already use it, you may find this discussion interesting:

**broken link removed**

http://dangerousprototypes.com/2012/09/29/pcb-manufacture-using-oracal-vinyl-film-laser-printer/

In brief, the video describes using vinyl for the backing, instead of paper. The question of whether it will melt on the laser fuser was not fully addressed, but several posters to that group claimed great results.

I use the photo method, make my transparencies on a laser printer, and then darken them with white board marker as described in my blog here at ETO.

John

 

[SwedishWings]

Just search for toner transfer. It's a process where the board outline is printed on paper and then ironed on the copper clad. If you are using photo transfer and are happy with it, I would stick with that. Toner transfer is much more finicky and can take a good amount of work to get right.

Ok. I think my process will work at least as well.

 

[SwedishWings]

If you decide to try toner transfer, and for those who already use it, you may find this discussion interesting:

**broken link removed**

http://dangerousprototypes.com/2012/09/29/pcb-manufacture-using-oracal-vinyl-film-laser-printer/

In brief, the video describes using vinyl for the backing, instead of paper. The question of whether it will melt on the laser fuser was not fully addressed, but several posters to that group claimed great results.

I use the photo method, make my transparencies on a laser printer, and then darken them with white board marker as described in my blog here at ETO.

John

Thanks, i got inspired by the video. I have to try it out! Though, i'm not sure where to get that vinyl they are using. Any ideas?

 

[jpanhalt]

Check out the Yahoo thread. Apparently, that was just the brand of outdoors vinyl he had available. Others in the Yahoo group claim to have used or were about to use other brands of vinyl.

Frankly, I would hold off on those experiments until later and just use the photo method you are accustomed to. My printer is 600 dpi maximum, and its transparencies are quite light. They will work as is with a few pinholes, but the marker I mentioned makes them quite opaque.

John

 

[SwedishWings]

They will work as is with a few pinholes, but the marker I mentioned makes them quite opaque.

Thanks for the idea about the markers, never heard that before. I suppose you refer to white board pens?

My solution have been to make 2 printouts, one mirrored, and then sandwich them together with the ink inwards. It requires film that handles the heat though. The HP film is ok, but cheaper film create alignment issues. It has another advantage; the film becomes much less sensitive to scratches.

I will certainly try your marker solution!

 

[jpanhalt]

Yes, I am referring to the easily erased white board pens with a broad tip. Here is a link t the blog: https://www.electro-tech-online.com/blogs/jpanhalt/213-improved-laserjet-transparencies-pcb.html

John