Electronics > Manufacturing & Assembly

Some reflow questions from a beginning flower

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I just bought a T962, applied some well know modifications and did a quick test on an old PCB with a TSSOP IC and just a stripe of solder paste (no stencil)
I did not expect good result, which was correct. Had to remove many shorts afterwards. (Tried it because I read other people do it as well)

Now I want to test with stencil, which leads me to some questions.

My idea is to make a (low cost, but testable) PCB design just for reflow testing. Order this design with stencil at one of the famous companies. I wanted to add at least a small BGA 0.5mm pitch, a QFN 0.6mm pitch and a TSSOP.
Parts already selected: I2C I/O expander, 16 bit buffer and an MSP430 microcontroller. Basically it is easy to create some microcontroller S/W which is capable of testing most of the connections. (was planning to route all connections via THT pin header as well)

Now my questions.

1. I am not the first DIY reflower, aren't there any similar designs/kits available just for reflow testing. I have the feeling I  am re-inventing the wheel.   
    One of my first target boards will contain a $120 QFN, I do not want to have to throw it away. 
2. I have not used a stencil before, how to avoid solder past 'move' when lifting the stencil, are there any tricks ?
    (e.g. if the PCB is fixed between some other pcb's, the stencil is taped on one side and you lift it from the other side. Will that work ?
3. Any DIY reflow experiences with 0.5mm pitch BGA ?
4. One of my real targets for reflow will be an RPI CM4 which has fine pitch connectors. Anyone reflowed this at home ?
5. Any other tips/suggestions are more than welcome  :)

[attach=2]First problem is to not apply too much solder paste.  You want to use thin stencils (I use .003" = 0.075mm) and reduce the size of the apertures to smaller than the pad size.  The smaller the component lead pitch, the more reduction you need to use to prevent solder bridging.
Use good solder paste.  Henkel GC10 is quite amazing stuff, sold in the US under the Loctite brand.  Accurate stencil alignment and accurate component placement greatly helps with the bridging problem.  Finally, proper reflow temperature profile is important.

I have used unframed stencils with good results.  I use masking tape to create a "hinge" on the edge of the stencil, so it does not move sideways when lifting.  I made a holder that clamps the edges of the boards with a step the thickness of the board, so the rest of the plate is level with the top of the PC board.  That is where I attach the tape to.
I have attached pictures of the frame and stencil setup.[attach=1]
I have not done BGAs with this technique, but lots of TQFP144 parts with 0.5mm pitch.

Hello @Prutser,

For the really fine pitch components you should lift the stencil vertical. Look for a stencil printer with this option.
The most used solder paste type is type 3. When apertures on the stencil get smaller you go to type 4 or type 5 solder paste.

Like @jmelson has already written, use a solder paste for fine components and optimize the stencil design.

Thanks for the replies and the pictures. Lifting the stencil vertically seems indeed the best option, however stencil printer prices start around 300,-  which I consider a bit too high for my occasional hobby use.
The main reason why I am starting with reflow is the fact that there are now parts which are impossible, or very difficult to solder by hand. When reflowing, I might just do only some ic's and connectors and still do the remaining components manually.
With manual placement you have to touch the parts anyway, so using a solder iron isn't that much extra effort. (and with not all parts on the PCB it's sometimes easier to perform some initial testing)   

One of the things to do on my list, was a carrier board for an Raspberry PI CM4 module. This module has 2 fine pitch 100pin board to board connectors. I was wondering how to (manually) position them correctly. I read some experiences on the net from people which had alignment mismatches (the module didn't fit) after assymbly in an assembly house. 
When I was looking up the assembly info from Hirose they adviced not to use 2 of those connectors on a single board :-(

Any experiences with multiple multi-pin SMT connectors on a single board ? 

FYI.....Henkel GC10 (Loctite) comes in two different sizes. So if you use this stuff (as I do) then make sure you order appropriately.

Solder bridges on reflow almost always down to bad application, too much paste etc.
Perfect that part and you could almost reflow with a cigarette lighter (not really!)



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