HAXLR8R // Blog // PCB Assembly 101

PCB Assembly 101

by Zach.

Electronics are inside nearly every consumer hardware device you buy today. They are the brains and guts that make our devices interesting. If you’re building a hardware startup, chances are good that you’ll need to design and produce your own custom electronics. If you have an electrical engineer on your team, chances are they know how this process works. If not, then this guide is for you.

The home of Haxlr8r, Shenzhen, is sometimes called the Factory of the World, and for good reason. You can’t throw a rock without hitting an electronics factory. The goal of electronics assembly is simple: connect all of your components together using molten metal, copper foil, and some fiberglass+epoxy to hold it all together. Then it starts to get hard.

This process is called PCBA (Printed Circuit Board Assembly). It consists of these main steps:

  1. PCB Production
  2. Solder Paste application
  3. SMT Pick and Place
  4. Infrared Reflow
  5. Inspection / QA
  6. Thru-Hole Component Insertion
  7. Wave Soldering
  8. Final Inspection / Functional Test

PCB Production

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PCB is short for Printed Circuit Board. This guy is the flat piece of fiberglass board that contains the traces that connect your components, as well as the pads where those components connect to the board. It consists of laminated fiberglass + epoxy for strength, conductive copper traces, and a (usually) green solder mask to keep the molten solder where it is supposed to be.

These boards are made using highly automated digital fabrication processes. You can have tons of different options, lots of different layers, and can generally go crazy here. The PCBA factory will typically use an outside company to fabricate your PCB (also known as a ‘board fab’). You produce these by sending GERBER files to your vendor.

Solder Paste Application

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The first step to building your electronics is to apply the solder paste to the board. This solder paste is made up of tiny balls of metal (solder) combined with flux (a chemical to help the metal melt and bond better). It is a grey paste and must be applied to the board in exact places and precise amounts.

In a professional environment, this is done with a solder stencil, a precision fixture, and an automated paste application machine. The solder stencil is a thin sheet of steel with tiny holes in it for the paste to pass through. The stencil is overlaid on top of the board, and paste is squeegeed across the stencil. When the stencil is removed, you are left with solder paste in exactly the right places.

SMT Pick and Place

After the solder paste has been applied, your board will move to the pick and place machine. This robot will automatically grab and position your components into the correct places so that they can be soldered to the board. SMT stands for surface mount components, which means that they are only soldered to the surface of the board. The vast majority of non-connector components these days use SMT technology. The reason for this is that SMT pick and place machines are extremely fast, accurate, and can generally run 24/7. Contrast this with human workers that are much slower, create errors, and need breaks… well you can see why SMT is the dominant technology of our era.

Infrared Reflow

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Once all of the components have been placed, the boards move to the reflow oven. This oven will head up the board until the solder melts, joining the components to the board. This process typically involves a conveyor belt that moves the board past a series of heaters that gradually heat the board up to the appropriate temperature, and allow it to cool down at a controlled rate. Often an overlooked part of the process, but the reflow part is very critical to creating good electronics.

Inspection / QA

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After the reflow process, you will have a board that can be tested. Common mistakes during the previous process will result in components that are not connected, shorts that connect portions of the circuit that should not be connected, or misaligned components. There are a variety of ways to check for this ranging from a quick look-over by a human, automatic optical inspection using image recognition, or even x-raying to look through components that might block your view such as BGA chips.

If errors are found, the board will be sent for rework or scrapped according to your quality plan. The sooner you find errors in your process, the sooner they can be fixed. This results in saved time and money. Test early and test often.

Thru-Hole Component Insertion

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Not every board can be designed with 100% SMT components. Certain things like headers, large components, and other devices only come in through-hole packages.

Most through-hole parts will need to be manually inserted by a worker on a production line. This can be a source for problems, so designers try to minimize this by using as many SMT components as possible. This production process is pretty straightforward. Typically a single worker will be responsible for inserting a single component. The board is then passed on to the next station until all through hole components have been placed into the PCB.

Wave Soldering

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Once the components have been placed, they must be soldered into place. This can either be done by an automated machine (wave soldering) or manually by using a good old soldering iron. Wave soldering is by far the preferred method, but has design requirements to work effectively. This process works by running the board through an oven where a wave of molten solder literally washes over the bottom of the board, soldering all the bottom pins en masse. This means you should typically try to have all your components located on the top of your board (both SMT and thru-hole). Certain non-critical SMT components can be placed on the bottom, but avoid placing small or delicate electronics on the bottom of the board.

Final Inspection / Functional Test

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The standard final inspection consists of what is called a functional test. A functional test will deliver power and signals to simulate the actual operation of the device. It will measure various electrical characteristics such as voltage, current, and signal output which will be compared against an acceptable range to determine if it passes or fails. The test fixture is a critical part of board design, and should not be overlooked until the last step. Most functional test fixtures consist of a ‘bed of nails’ which is a series of spring-loaded pins that connect to test pads on the PCB surface to inject power, signals, and to record data for test purposes. A good factory can help with the design and production of this test fixture.

After your electronics pass the functional test, they are either assembled into the final product, or packaged up for shipment. I hope this post has shed some light on the somewhat arcane art of making electronics. If you’ve got a great idea for a product, don’t hesitate to sign up for the HAXLR8R class of 2013.