Manufacturing test is a tricky optimization problem. Too much testing can drive up product cost through capital expenditures, development time and test operator costs. Then again, too little testing can lead to higher return rates, angry customers and damage to the brand.
For the new wave of Internet of Things (IoT) products, designers may be tempted to overlook manufacturing test in their haste to get new products to market, but it’s not advisable. Today’s manufacturing test methods and equipment can move as quickly as IoT products and help avoid returns and other problems. Investing in manufacturing testing is easier than ever with low capital costs and incredibly fast setup times.
Because IoT products are quickly evolving, their development and revision cycle is necessarily short. Well-architected IoT products will be able to have their features and applications evolve with changes to online interfaces or remote firmware upgrades, but even these upgradable systems will need to have manufacturing systems that can rapidly scale up and down to enable high-volume production. Maintaining low product cost — critical for the IoT — will require low-cost, flexible and effective manufacturing testing.
Developing the System
It’s not unusual for the development of these manufacturing test systems to become a bottleneck in rapid IoT production cycles. Nevertheless, you should be testing every unit and not leaving test development to your manufacturing partners. Effective test design requires integration with the product design and a deep understanding of the product functionality. If you plan testing early, it will be easy, cheap and incredibly valuable in optimizing your design. If you don’t, you’ll spend time and money later.
Designing manufacturing test systems doesn’t have to be hard or expensive, nor should it just be a copy of the engineer’s validation setup. In a manufacturing test environment, there are many special considerations:
• Dirty power
• Electrostatic discharge (ESD)
• Bad units
• Flexibility and scalability
In a lab environment, engineers can trust fancy $10,000 power supplies to provide stable voltages accurate to the 100µV which are carefully current limited and well isolated from power line noise. IoT products are mostly battery-powered devices running from either small-volume LiPo or coin cells. Since their low cost and nearly-disposable nature doesn’t allow for expensive input power protections, poorly controlled power during manufacturing could be potentially destructive to an IoT product.
Unfortunately, manufacturing test systems can’t scale to high-volume production with $10,000 power supplies connected to every unit-under-test (UUT), so you need to design your test system to survive, handle and clean up dirty input power from low-cost supplies. Even with solid Standard Operating Procedures and training procedures, factories move fast and mistakes happen, but a manufacturing line should not stop simply because something was plugged in backwards.
To minimize these issues, test system design needs to handle potential mistakes like over-voltage, under-voltage, reverse-polarity, noisy grounds and even high-voltage ground lifts. The cost to add these protections to a test system is relatively minor and enables the use of low cost, readily available AC/DC power supply “bricks.” The capital equipment cost savings of replacing bench-top power supplies with low-cost power modules integrated directly into the test fixtures — while maintaining programmability, current-limiting and voltage precision — can be in the millions of dollars.
Your product will likely never see an ESD environment as harsh as the manufacturing line. Factories are full of people moving around, plastic product handling trays and moving conveyor belts — all of which are often coupled with broken grounding lines. This translates to some harsh ESD situations, and at many stages in the manufacturing process products lack final protective housing.
Test systems need to both survive in these destructive environments as well as isolate the UUT from harm. Adding high-speed transient voltage suppressor (TVS) clamping diodes, ferrite beads and chokes and resistor-capacitor filters to every interface with the outside world can harden your test system. Again, it’s cheaper to over-compensate in the test system design than to add all these components to your product’s bill of materials cost.
Bad Apples in the Bunch
Not every unit produced will be good. Short circuits on power planes, bad parts, wrong parts, incorrectly oriented parts, other AOI/AXI escapes and even wrong designs will find their way into your test fixtures. To protect against breakdowns, test systems should be designed to have every interface to the UUT protected up to the highest voltage present in the system, be current-limited, handle direct shorts to ground and be able to survive ESD and other surge currents. Inductive voltage spikes from long cable runs are frequently overlooked, especially in long power supply wires, including those from hot-pluggable interfaces like USB. Your average USB hub, if it has any current-limiting at all, will use slow poly-fuses. A bad UUT may be able to draw several amps before the poly-fuse kicks in. An unplug-event with those currents will destroy most cheap USB hubs and stop your manufacturing line.
Flexibility and Scalability
A manufacturing test design should speed up product cycles and ramps, not slow them to a crawl. Standard test fixture approaches rely on slow and unreliable wire-wrap assemblies that rarely work on the first try. Bringing up and debugging your product should not start with second-guessing and rebuilding your test system. After prototype builds, assembling more wire-wrapped fixtures will likely be the tightest bottleneck to ramping production volume. Think about it this way: your product doesn’t use wire-wrapping for many good reasons, so why would your test system?
Bottom line: assembling a test system with modern assembly techniques, modular interconnected pieces, embedded power systems and embedded microcontrollers with real-time operating systems can allow you to quickly debug production issues, change your test coverage as your product and needs evolve and — most importantly — scale to meet growing needs.
Justin Gregg is CEO and principal engineer at Acroname.