It sounds overly complex, but yes, there are a lot of things to get right if you want it to work 100% reliably. This prevents excess paste from lifting and floating the part away from the pads at the edge of the part. Most small area pads should get a solder paste stencil aperture equal to the pad size, but with larger pads, like die attach pads, a large aperture should be broken up into smaller apertures that only cover a percentage of the total area, perhaps around 60%. Pricing and Availability on millions of electronic components from Digi-Key Electronics. The Hakko has one built in that it uses to control the temperature profile, but you can use your own if you're using a random oven.Īnother reason why a stencil is so great is that by designing the footprint properly, you can deposit precise amounts of solder paste, and reliably handle parts with die attach pads like DFN and QFN and SO-8 parts with DAPs. NC191LTA10 No-Clean Solder Paste Bi57Sn42Ag1 (57/42/1) Syringe, 0.35 oz (10g), 3cc from Chip Quik Inc. There's definitely some trial and error involved and I would strongly suggest that you tape a thermocouple to the board so you can see what is happening. With this paste, taking too long to get to reflow causes graping and generally non-smooth and shiny joints, but getting there too fast prevents the flux from fully activating and steam/gases from baking out. Once it's barely starting to flow, I use a hot air wand to bring each part of the PCB up to reflow temperature. It gets the PCB up to 180✬ as quickly as it can, which ends up taking around 3.5-4 minutes to get to 170✬, as read from a thermocouple taped to the PCB with Kapton tape. I use a Hakko infrared pre-heater running a program that seems to work well via trial and error. Remove three sides of the tape, lift the stencil as if it was hinged off of the fourth piece of tape, remove the PCB, and then place the parts and reflow. The process: put the PCB on the bench, immobilized between some taped down corner mounts, put the stencil over the PCB, line it up, tape the four sides of the frame down to the bench, apply paste with one pass of a stainless steel squeegee. I can only find this paste in 500g jars, but it keeps well in a refrigerator, much longer than the expiration date suggests. I get the version pre-tensioned onto an aluminum frame. I'm using a 0.12 mm thick stencil that I order with PCBs from PCBway. Early on, I tried using a syringe, but it's just too time consuming and sloppy. It's a 63/37 lead paste, but it flows easily, and behaves really well. I like Kester EP256 solder paste for use with a stencil.
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