The Role of Programming and Testing in PCB Assembly

A small aerospace printed circuit board contains enough wires to stretch across a football field. A PCB in a modern industrial hardware can contain as many as 4000 components. A PCB in an electronic device might be as thin as .2 millimeters, contain parts the size of a flea, and still have multiple layers managing multiple currents. Whether it’s mission critical embedded hardware or capital-intensive manufacturing equipment, the tiniest PCB assembly flaw at the very least prevents your equipment from working, at the most, may destroy it.

Automating Programming and Testing in PCB Assembly

Accounting for the minute details involved in the assembly of a quality PCB requires comprehensive attention to detail, repeatable testing, and the elimination of human error through industrial automation. Automating a combination of testing methods throughout the entire PCB Assembly process can prevent costly errors magnified in mass production. Programming traceability into the testing architecture keeps your PCB consistent, functional, and cost effective. 

Consider the numerous ways PCB assembly can go wrong on a production line: 

  • Part Misplacement- Your PCB layout plots every individual resistor, inductor capacitor, diode, transistor, transformer, integrated circuit, battery, connector, sensor, switch, and oscillator on your board. You must manage not only the correct inventory of parts used in your PCB, but each part also must be placed in the exact spot on your board. A single missing or misplaced part on a board can break your hardware. 
  • Part Quality- Determining whether parts added to a board are up to date or in good condition can be the difference between a solid product and a costly mistake. PCB parts can expire. A new reel of PCB parts might have a single damaged part. Poor storage processes can prematurely ruin materials. A poor-quality supplier may not disclose issues with storage and shipping. One damaged or low-quality part might cut the longevity of your hardware below minimum expectations.
  • Exposure- A perfect PCB can be ruined by exposure to temperature, moisture, dust, and chemicals. Even minute amounts of foreign material on your board can cause anything from premature electrostatic discharge (ESD), surges, overload, or power disconnection.
  • Soldering Errors- Joining the metal connections of a PCB to parts requires permanent metal soldering for bonding. Too little soldering could mean a poor bond between the PCB and your parts. Too much solder, or ‘Spattering’, could lead to components floating above your PCB without electrical connection or ‘tombstoning’ your components, having a component lift off one side into the air during reflow.

The complexity, speed, monotony, and repeatability required to assemble a large quantity of quality printed circuit boards requires automation to meet those targets. 

  • Programming- During the layout stage the electrical engineer utilizes your schematics, gerber files, and bill of materials (BOM) to create programming that places every part, maps every connection, and plots every testing point for automated confirmation. This virtual model is plugged into a series of assembly line style machines capable of automatically referencing that model and assembling your boards.
  • Conveyors- A series of conveyors programmed to maintain, halt, and resume speeds depending on the process necessary moves your boards from one assembly stage to the next. They create a buffet of parts to keep supply constant. They also allow open areas for human intervention, like removing articles for offline testing and verification.
  • Solder Application- A machine overlays a premade stencil over your PCB that only exposes the areas that require solder paste. A machine automatically applies the paste and scrapes off excess. This allows a mounting surface targeted for the variety of parts needed on your PCB.
  • Pick and Place- A high speed pick and place machine rapidly places the parts on your mounting points in the correct orientation. Some machines can place tens of thousands of parts per hour without error on printed circuit boards.
  • Reflow Soldering- Up close, solder paste is composed of a film containing tiny metal balls. The paste is strong enough to hold your parts in place for the pick and place and conveyors but is not a permanent bond. To complete permanent metal joining to your boards, your board is baked in a reflow oven. The oven carefully melts the solder paste into permanent metal joints. The reflow oven program precisely manages the ramp up in temperature and cool down to not exceed the temperature tolerances of your PCB components.
  • Depaneling- When a single panel containing multiple PCBs moves through the assembly line, the depaneling machine safely and carefully removes the individual PCBs from the larger panel. These PCBs can be removed with physical actions like punching, cutting, stamping or with lasers.
  • Testing- Testing machines work in concert through the entire PCB assembly process. Some like In Circuit Testing or Burn in Testing may only occur once and at a specific time during the PCB assembly process. Others may happen multiple times along the assembly line. Automated Optical Inspection (AOI) for instance commonly happens after soldering and reflow.

Automated PCB Assembly Testing Methods

When your engineering team designed your printed circuit board, they put markings on the silkscreen and test points into the PCB layout. They programmed the 3D layout, a series of quality control instructions specific to the board and added in a series of dedicated testing machines to perform swift automatic quality checks on the boards. These tests work as separate stations along the assembly architecture. Not all of these types of testing methods are used when checking the quality of a PCB, but one type of test can’t cover every testing concern. 

Burn In Testing

Usually done in the prototyping stage, this is a stress and longevity test of the PCB. Power is supplied either at the extreme upper limit or above. Depending on the product’s functional environment, additional exposures such as temperature or humidity are added to the process. The PCB is programmed to run excessive cycles, at maximum speeds, and for excessive lengths of time. This may sound like an attempt to break the PCB and it is. Determining when, how, and how common the breakdown happens during Burn in Testing allows early detection of manufacturing flaws and an understanding of the common weaknesses of the board. The PCB engineer can then make updates, upgrades, and mitigations that support the board’s longevity.

Automated Optical Inspection (AOI)

This type of inspection involves a series of focused snapshots and camera assisted sensors. These are often placed in multiple stages of the assembly. The AOI machine compares the physical board to the expected 3D rendered image for visually improper placement of parts, visual indicators for defective parts, soldering errors, defective boards, and cleanliness. Boards that pass AOI can move on to the next stage of assembly or to a non-visual form of inspection.

In Circuit Testing (ICT)

Usually near the end of the PCB Assembly line, the In Circuit Testing machine physically checks independent testing points and components on the board all at once. Sometimes called the ‘bed of nails’ for the similarity in shape of the distribution of probes, this machine presses down a platform of probes into the board pins and testing points. A preprogrammed testing sequence verifies the basic qualities of the board and checks for unexpected shorts, opens, resistance, surges, and other defects that can only be detected by contact and activation. Once the board quality is verified, the board moves on to the next stage or gets flagged for rework or discard. An in circuit testing machine may test one board per minute, so it’s ideal for mass production. 

Flying Probe Testing (FPT)

Perfect for low volume PCB production or prototypes. The typical flying probe machine utilizes a gantry systems that runs a single axis series of probes designed to move from one test point to the next. One to four movable and fixed probes ‘fly’ back and forth across the machine to individually test components. While the flying probe has a lower upfront cost, less custom parts, and swifter programming, the speed is much slower, with a small board taking as long as fifteen minutes.

X-Ray Inspection

While testing methods like ICT and Flying Probe can easily test Surface Mount Technology  and exposed testing points, complex boards with multiple layers and pins under parts might need X-Ray Inspection. An X-Ray Inspection machine can image the inner workings of boards including inner layer electronics, inner solder joints, high density component layout, and layer specific assemblies. If you have a dense and complex arrangement of board layers, this is the only way to visually confirm the quality before functional testing.

Functional Testing

While ICT, Flying Probe, and X-Ray can automatically find defects in your boards during assembly, they lack the ability to test the entire board working in concert. Functional testing occurs at the end of the run, where your engineer hooks up your board to an automatic functional testing machine. These machines run your board in a simulation made to mimic the inside of your hardware. If the board passes a series of preprogrammed tests from signal integrity to product purpose, it moves off the line for installation in the final hardware product.

Maintaining quality over thousands and millions of printed circuit boards requires efficient programming and automated testing. Tell us more about your project, schedule a virtual meeting, or call (262)-622-6104 to learn how DEVELOP LLC can bring your hardware to mass production.