Populating the PCB

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Revision as of 16:49, 3 April 2019 by Aplstudent (talk | contribs) (Optional: Surface Mount Devices (SMDs))

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If your board only contains through-hole components (components with wires or leads that will mounted through the board) you can ignore the section on Surface Mount Devices below. If your board has surface mount devices then you will want to read the section about surface mount devices before you attach any components.

Once the new clean board has had the acetone washed off, any through-holes need to be drilled.

Use the drill press at maximum speed with the miniature carbide drill bits. WEAR GOGGLES, these bits are notable for shattering. Push the bits through the board slowly - they are going at 1/30 of their intended speed in our press! Typical drill holes are .028 for wire pins, .035 and .042 for normal device pins. Be sure that your drill bit matches your component pin size, if the holes are too small the component will not fit and if the holes are too large you run the risk of drilling through the copper that will allow you to solder the pin to the board.

The board obviously will not have plated through holes the way professionally manufactured boards do. Any connections must be soldered on both sides of a through-hole device, and any vias must have wires run through and soldered.

If you have not soldered before, you can read this article, and/or ask someone for help. Soldering isn't wildly difficult but the process has a few nuances that are important to understand to be successful, namely how to ensure that the soldering joint is properly contacting both the wire and the PCB.

Optional: Surface Mount Devices (SMDs)

WARNING: SMDs require the use of solder paste, which may contain lead in an easily indigestible form. You should take take extreme care when using solder paste. Wear gloves and clean any surface that the solder paste comes in contact with. Try to avoid any contact between the solder paste an your skin.

Surface mount devices are components that are soldered directly to the surface of your PCB. To attach these, you must use solder paste and a reflow oven, in our case we will be using the "Puhui infrared IC heater", to melt the solder paste.

The heating process requires placing the whole PCB in the reflow oven. Since the melting point of the solder is around 183 degrees Celsius, you want to be sure that you either attach the SMDs before any other components or know that the components already attached to the board can withstand the 183 degree temperature without being damaged. To be safe, it is always better to attach SMDs before attaching any through-hole components.

Applying Solder Paste

The first step in attaching SMDs is applying soldering paste. Applying the solder paste is most commonly done in two ways, with a stencil or by gently dabbing solder paste on the PCB. Using a stencil can become necessary when you have many SMDs or if you have surface mount integrated circuit that requires multiple pins to be attached. For larger or fewer SMDs you can apply the solder paste directly to the board without a stencil by dabbing the paste on SMD's board location. When applying solder paste without a stencil it is better to err on the side of less solder paste, rather than more.

Applying Solder Paste with a Stencil

If your project requires a stencil that does not exist in the lab already you should talk to Professor Boggs about how to obtain one. The fabrication of a stencil is beyond the scope of this tutorial but as a reference you can look at Seeedstudio's stencil fabrication, or talk to people in the maker's space or machine shop.

Assuming you have the stencil, your goal is to evenly spread solder paste over all the surface mount locations on the PCB at once. First you want to secure your PCB to a flat surface and make sure your stencil is securely and flatly attached to your board. You want to prevent any lateral movement of the the PCB or the stencil as you smear soldering paste across the stencil. Once your PCB and stencil are secured to each other and flat against each other, apply a large amount of solder paste to the area above the holes of the stencil (see the picture below). Next, in one motion you want to drag a blade (or other flat object) across the stencil to smear solder paste over the holes. Proper execution of this motion results in an even distribution of solder paste across every hole in the stencil. Unsuccessful solder application can be rectified by washing away the solder paste and trying again.

Attaching SMDs

After applying solder paste, you can begin attaching SMDs to your board. Due to the small nature of SMDs and the precision necessary to attach SMDs to the PCB, use finely tipped tweezers to place your components on your PCB. After placing all your components on the PCB carefully bring your PCB to the reflow oven.

Using the Reflow Oven

Read the brief manual taped to the top of the reflow oven before you use it. The manual will tell you which program you need based on the kind of solder paste you are using (the solder paste currently in the lab is 63Sn/37Pb). As mentioned before, double check that the reflow oven's heat will not damage any components that may be attached to your PCB.