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Stencil-PCB Alignment Mehods - Printing process

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Stencil-PCB Alignment Mehods - Printing process | 9 December, 2011

Dear All,

I am a totally new to the SMT process. In fact, I just had an overview course regarding the whole process of PCBA manufacturing and I have know a bunch of questions in my head.

Most of them are linked to the solder paste printing process. Indeed, this step sounds to me essential and I didn't understand what were the techniques used to align the stencil with regard to the PCB (or to align the PCB with regard to the stencil).

Which features and processes are used to make it happen?

Many thanks in advance for your answers.

Kind regards Alexander

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Stencil-PCB Alignment Mehods - Printing process | 9 December, 2011

Can you use the equipment manual as the basis for learning how to use the machine?

Each model of printer has different alignment techniques. We don't have a clue of your model type. So, we'll approach the question from its most basic point.

Registering Board: 1. Lift stencil frame to up position. 2. Position clear Lexan sheet provided using the two brackets at rear of Print Deck. 3. Lower stencil frame to "down" position 4. Use squeegee to apply paste over stencil to clear sheet. 5. Lift clear sheet enough to "roughly" place circuit board on Print Deck (match up pads on board to paste on clear sheet). Remove clear sheet without moving board. 6. Slide Board Stops to touch all edges of PCB. 7. Tighten Board Stops in their slots using a hex key. 8. Adjust height of print deck beneath frame with Z axis knob. The tensioned* stencil can be flush on the board or slightly above the board surface. 9. Using the X,Y and Ø -axis adjustment knobs on the front and sides of the machine, register the pattern of the stencil apertures to the pads on the board. Final registration can be done most accurately by looking directly down on the stencil while moving the plate. NOTE: For pin positioning of circuit boards contact factory for recommended methods. *If using a screen or untensioned stencil, it's important to support the area around the PCB with boards of identical thickness to prevent screen damage.

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Stencil-PCB Alignment Mehods - Printing process | 11 December, 2011

You can also use a portable measuring microscope to check your alignment before each print if you don't have some form of semi automatic or automatic alignment method. They look something like this

If you are just starting out in the game this is one of the first things I'd purchase, particularly if you are using a clam shell printer or lower end printer with mechanical alignment only. You can check the alignment before each print rather than relying on the accuracy of the pcb outline or tooling holes etc. and you don't have to clean up any of that messy mylar print. Cool huh? You want something with around 5>7mm FOV with graduations of 0.05mm. They'll cost around $100 - $150. Fun for measuring other "stuff" as well. Enjoy yourself amigo and you're right, printin' is REAL important.


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Stencil-PCB Alignment Mehods - Printing process | 16 December, 2011

About the automated printing machines:

They are all a little different, but the alignment part of the process is basically the same.

I don't know if you have ever seen an automatic printer, so I'll do my best to explain it.

The PCB rides into the machine on two edge rails. It is positioned (either by hard stop or sensor and software).

Next the PCB is clamped either by edge clamping or by a vacuum plate from below.

Next a camera with upward and downward looking lenses finds the pre-programmed alignment features. Then these (two at least) features are aligned with each other.

Next, the camera moves out of the way and the board and or stencil is moved to print height. After that, the Squeegee does it's thing.

About the Alignment features, They are typically round dots in opposite corners of the PCB. We call them fiducials or fids for short. There are often fids close to fine pitch pads as well for local alignment. If the PCB was designed without fids, most printers can be programmed to use pads for alignment.

This Youtube link shows the alignment process on an automatic printer:

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