Setting up an SMT Assembly Line

The LumenPnP solves one part of making your own PCBAs, but there's more to the process than just placing parts! In this article, we'll show you how to set up the four parts of a medium scale SMT line.

Pasting

The first step in getting your boards populated is applying solder paste. There are a vast array of different kinds, but there are two that are a great starting point:

This paste is a great starter paste. It is low temp, meaning a smaller or less powerful reflow oven should be able to handle reflowing it without an issue. 

This paste is excellent. It's a bit more expensive, but has excellent properties and greatly reduces the chance of bridging or cold joints. This is what we use at Opulo for all of our boards.

As for your stencil, we find that the stencils you can get made at your board shop are excellent quality, and work great for a mid-scale line. We get our stencils made from JLCPCB. Getting your stencil "framed" means the steel stencil is mounted on an aluminum frame, which makes it much easier to use. We highly recommend the "Electropolishing" finishing option, as it's only a few dollars more, and makes the paste release from the stencil much easier.

The last step in pasting is getting a paste printer. A paste printer is a jig that perfectly aligns your stencil to your board in a repeatable manner, making it really easy to line everything up.

At Opulo, we use a couple 3040 Solder Paste Printers, and they work well for our purposes. Once they're set, they hold their alignment well.

Placing

The LumenPnP is a workhorse for getting boards populated. After configuration, it's just a matter of placing your pasted board onto the machine, hitting "run", and letting the LumenPnP populate your board for you.

The LumenPnP can populate passives as small as 0402, at up to 800 chips per hour. It's capable of placing large ICs as well, including TQFP-100 chips with 0.5mm pitch leads, and even QFN parts with 0.4mm pitch pads.

A pick and place is the beating heart of an SMT line, so it's critical that you have a reliable, precise machine that you know is producing accurate placements.

Reflow

With your board pasted and placed, it's time to reflow! You can reflow your boards in a number of ways, but these are the options that we think are best:

This is a kit sold by Seon the Unexpected Maker which lets you modify an off-the-shelf toaster oven into a reflow oven. Many folks recommend using a Black & Decker oven with a convection fan for this. So far, this is the best option we've found for reflow; it works fantastic. It's what we use every day to make our boards.

If you only have SMT components on one side of your board, a hot plate is a fantastic option. Many of these won't actually go through the proper temperature profile that many pastes are designed for, but they'll still perform well after a bit of timing and temperature tuning.

This oven seems like it would be a great option, but frankly it's not quite there. There are even upgrade guides to try to make up for its shortcomings. We bought one of these and have gone through a number of the community modifications, and it works quite a bit better after performing them.

Through-Hole

Soldering through hole parts can be an incredibly time consuming process. Dip soldering is the best approach we've found to make it easy and consistent. Dip soldering is the process of populating all the through hole parts on your board, applying flux to the board and component leads, then dipping the whole assembly into a pot of molten solder.

Before dipping in solder, we give the populated board a dunk into Kester 959T Flux. This flux has a very low viscosity, so it will find all the surfaces that need to be soldered.

We use a CM282 Solder Pot for our boards, and it works wonderfully. It's plenty big to fit any of the panels we make at Opulo, and it heats up in a reasonable amount of time.

Wrapping Up

With all your components placed and soldered, you can perform any quality control checks, programming, and functional testing for your board.

We hope this helps illustrate how you can easily assemble your own boards in-house! If you have any questions about this process, or would like more information about getting started, please feel free to email us at sales@opulo.io.