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19th June 2018

Our guide to how electronic products are manufactured - Part One

Your product idea has been realised into one or more electronics printed circuit boards by a competent team of engineers.  It is worth understanding the various stages of the process for manufacturing and testing the boards.  Having an awareness here helps your supplier selection and due diligence processes to ensure you have consistent and acceptable quality for the products. Here’s our guide to how electronic products are manufactured.

Bare printed circuit boards

When purchasing in high volumes printed circuit boards (pcb’s) are often imported from Asia. It is usual for the manufacturers to test these boards.  The test method chosen will depend on production volumes and the stability and longevity of boards.  Where a design is expected to run in large volumes and is not likely to change the manufacturer may make special tooling with probes on each desired test location called a bed of nails tester, this speeds up the test as many nets can be tested simultaneously.

For lower volumes, or those subject to frequency change, the boards can be tested with flying probes. These probes mean you don’t require tooling as they move around the board.  This results in slower testing and is a trade-off of costs due to test speed vs tooling costs

The test methods used are the same for both where resistance and capacitance is measured looking for open, short and high-impedance traces.

Simple single sided boards can alternatively be optically inspected instead as all of the tracks are visible.

I believe that all bare pcbs should be tested. Failures here can be time consuming and difficult to diagnose and may not easily be repaired. The cost of a faulty pcb extends way beyond the board itself as the components fitted to the board are likely to be wasted. I would expect decent pcb manufacturers to do this as standard.

Automatic Optical Inspection (AOI) – solder pasted boards

During the manufacturing process solder paste is usually screen printed onto printed circuit boards directly onto each component pad. Solder paste is made up of small balls of solder suspended in flux and other chemicals. Amongst other issues too much solder paste can cause solder bridging, too little can cause open circuits.

You will find that a good Electronics Manufacturer should have this process under tight control. It shouldn’t be necessary to ask them for solder paste inspection. Some of the more modern paste machines include optical solder paste inspection.

AOI – placed components

The next stage in the surface mount board manufacture process is placing components onto the board into the solder paste.  Vacuum nozzles pick up each component and place them onto their respective locations in the correct orientation. These machines are rather inventively called pick and place machines.  Many components are polarised and must be placed the correct way around in order for them to work.

Many pick and place machines include some level of optical component verification. This can include orientation and identification and will discard components which fail the tests.

Some contract electronics manufacturers (CEM) pass placed boards though automatic optical inspection to ensure all components are present, correct and the right way around as its much easier to rectify any issues before the solder paste is melted into solder.   There will be some time associated with set up of the machine for the inspection but this should be part of the CEM process and isn’t something you would usually pay extra for. Inevitably these costs will be baked somewhere.

AOI – after reflow

The printed circuit boards with components placed into wet solder paste are then passed through a reflow oven.  The reflow oven takes the boards through a number of heating and cooling stages.  The board is first heated up at around 1-3°C per minute to a temperature of around 150°C. It is then held at this temperature for around a minute which lets the flux in the solder paste activate and ensures all components have achieved the same temperature.  Heating boards much quicker than this can expose components to thermal shock and damage them.  After the soak period the board is then heated up to a peak of around 220°C where the solder becomes liquidus and flows to connect the device terminals to their pads. This is followed by a cooling stage where the solder solidifies and the board cools back down.

It is possible for a lot to go wrong during this point, especially if previous paste and/or placement steps have not included any inspection.  There may be missing components, dry joints, solder bridges between pins, components twisted off pads through surface tension differences, components which have been shadowed by a larger component and had insufficient heat to reflow or even components which have stood up on one of their terminals often caused by copper in-balances in the traces connecting the components making one end cool down quicker than the other.  Design for manufacturing reviews can help avoid issues due to tracking and component placements.

Automated optical inspection at this point can check to see that all components are fitted, are the correct parts and have no visible defects.  This is a machine learning process so a quantity of boards will need to be tested to program the inspection.

X-Ray

The drive for smaller devices with more functionality has led to device packages being made smaller and smaller.  Many modern ICs have terminals, or pins, underneath the devices in Ball Grid Array (BGA), Quad Flat No-Leads (QFN), Land Grid Arrays (LGAs) or Wafer Level Chipscale (WL-CSP) packages which allow for higher performance and tighter adjacent placement.  For these devices AOI is not enough and x-ray inspection can be turned to.

X-ray machines can be placed in-line and used to check every board or can be an offline activity.  X-raying boards can be slow and may be best for first-offs when setting up production or for analysing failures.  In practise you will find that these leaded devices can be reliably placed once the initial setup has been completed.

In part two we will cover the steps following manufacture for functional test, programming and setup of manufactured boards.