Usos Del Stencil

8/14/2019 Usos Del Stencil

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SparkFun Electronics Stenciling

Materials by Bob Hunke & Linz Craig



Why Stencil?

• Smaller footprint means smaller board

• Tight pitch components

• Multiple boards at a time

• Greater efficiency

• “Hidden” connections



Materials Needed

• Stencil

• Paste

• Reflow capability

(oven, hot plate, iron)

• Stencil frame

• Squeegees/putty knife



Plastic vs. Metal Stencils




Cost• The metal stencils SFE gets are laser

etched & sold by the 15” x 15” sheet at

$125

• Plastic stencils are $25 plus shipping per

board




Ease of Use• Make sure that your stencil extends over

the end of your frame or framing scrap

PCBs

• Stiffer stencils are easier to pick up off of

the board, hence metal is easier to workwith




DIY• Cut your own plastic stencils fairly easily

• Chemically etch your own metal stencils

not so easily• Machine cut your own stencil if you’ve got

the hardware and software (this is

becoming more and more common)




Quality of work

• Metal stencils tend to make for betterpaste placement



Paste Types

• Leaded vs. Lead free

• Flux: Clean vs. no clean

• Size: SFE uses Type 3, average soldersphere of 36 microns

• Five-ball rule for choosing type:

Smallest component footprint should be nosmaller than five times the width of average

solder sphere



Type 3 Paste Cost

• SFE, leaded: $9.95 for 50g

SFE, leaded Lead free, $63 Syringe style

• Larger quantites: 500g $100 - $150



Paste Storage and Handling

• Store your paste ina cool place such

as a refrigerator• For short term

storage room

temperature isokay



Applicators: Squeegees

• Professional,designed

specifically forpaste, machineready

•

Possible tochange blade

• Expensive($35 - $350)



Applicators: Putty Knives

• Inexpensive($1 - $20)

• Sharp edge (maydamage stencil)

• More flex than

Squeegees



Safety Concerns

• Sharp edges

• Lead, tin, silver,copper

• Eye irritant• Extreme temps

(re-flow)

• Fiberglass shards

• Occupational hazards(Paster ’s Elbow)



Setup

• Frame your PCB



Setup

• Apply paste to your applicator evenly



Setup

• Align yourstencil

• Apply steadypressure onone side

• Make surethe surfaceis flat & even



Apply the Paste

Proper grip 15 – 45 degrees Even

distribution

Cleanup 60 - 80 degrees Hold & lift off



Populating

• Tweezers and steady hands

• Placement diagram (build sheet) to makesure everything goes onto the right spot

• Careful with polarized components

• Start with lower profile components first



Populating

• Small margin of error (surface tension)

• Don’t smear the paste

• Nudge with tweezers, don’t push down



Reflow: Professional re-flow oven

• Big and costs a lot ($1500 used on ebayto $50,000 and up)



Reflow: Desktop re-flow unit

• Small unit, IC controlled cooking

• Mid range price ($250 to $800)



Reflow: Hot plate

• May yellow silk screen

• May cook unevenly

• You have to time it yourself

• Cheap ($40)



Reflow: Soldering Iron

• Can be difficult to reach components

• Useful for rework and PTH

• Price ranges ($10 - $200)



Reflow

• Heat sinks, PTH and heat sensitivecomponents should be added after re-flow



Paste Cleanup

• Paste isn’t cheap, save paste by scrapingit back into your container

• Clean off your stencil and tools with wateror isopropyl alcohol



Creating a PCB and Stencil

Layout Software• Eagle: www.cadsoftusa.com

Free trial download available, robust, what SFE uses

• KiCad EDA: www.kicad.sourceforge.net

Free opensource schematic & PCB layout software

• Fritzing: www.fritzing.org

Free educational virtual prototyping tool, only one

layer of traces available, great for beginners




Gerber File Formats• .GTP (top paste –aka stencil file)

• .GTO (top overlay –aka white ink)

• .GTS (top soldermask)• .GTL (top layer –aka actual copper)

• .GBL (bottom layer –aka actual copper)

• .GBO (bottom overlay –aka white ink)• .GBS (bottom soldermask)

• .TXT (drill –aka any holes in the board)




or Make Your Own• Ferric Chloride etching:http://www.instructables.com/id/Making-stencils-for-solder-paste-at-home/

• Laser Cutting:http://www.zblob.com/solder_paste_stencil.html

• Milling:

http://www.millpcbs.com/phpbb3/viewtopic.php?f=12&t=36

• Photo etching is also a possibility



More Info

• Stencil manufacturers:

Ohararp.com, Soldermask.com, Pololu.com

• Paste manufacturers:

Techni-tool.com, Kester.com, Zeph.com

• Squeegee manufacturers:

Ntscope.com

• PCB manufacturers:BatchPCB, Gold Phoenix, Advanced Circuits, PCBUniverse



Questions?



www.sparkfun.com

6175 Longbow Drive, Suite 200

Boulder, Colorado 80301

Usos Del Stencil

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