Printed circuit boards today are loaded with powerful digital and analog chips along with complex components into smaller, denser packages than previously possible. The boards include advanced digital signal processor (DSP), radio frequency (RF), and mixed signal chips in tightly packed, high-layer counts. But increased board densities create a higher probability of defects and faults, which lead to lower yields for some products. Consequently, demands from the medical community have made the pursuit of building in greater product reliability an ongoing process.

Defects and faults are almost synonymous. One leads to the other. For example, a digital IC-output pin that doesn't toggle correctly is a defect that results in a fault. In-circuit (ICT) and functional tests (described later) detect faults. However, there can be defects that don't show up as faults. These include insufficient or excessive solder, misaligned components, marginal joints, and open power pins.

Other component-level defects show up only at the end of the manufacturing process. For instance, before entering reflow stages, a misaligned chip may not self-align in the reflow oven. Also, insufficient paste can lead to a defective solder joint.

The workflow for companies building PCBs heads off these problems in four broad stages. They include built-in quality control (QC) throughout manufacturing, first-article research, thermal profiling, and key testing procedures. Omit one stage and you reduce your chances of hitting expected reliability levels.

The four stages

Design, fabrication, and assembly of PCBs each see a range of QC operations. There are 15 or more QC measures applied from start to finish.

The first QC stage verifies that solder paste is dispensed on each PCB pad. An SMT printer dispenses solder and a vision system makes sure every part of the surface mount has paste on it.

First-article research refers to engineering investigations of the first OEM PCB off the line. Contract manufacturers that makes the boards usually consider each design as custom. First-article research boosts reliability by ensuring component values, orientation, and polarity are correct and all power supplies, cables, and mechanicals are in place before shipping the product.

First-article research guarantees all design questions are answered and thermal profiles properly created. This gives OEMs a chance to test and debug the first-article board before building and shipping products. Changes to the original design or post debug fixes can also be included at this stage.

Thermal profiling details temperatures in a solder reflow oven. A board's complexity and construction dictate what temperature profile works best while the number of components on a board determines the profile complexity. Construction characteristics are studied by reviewing internal construction and stack up of bare PCBs, as well as material composition of components on the board.

Profiling has two primary objectives: Determine the process settings for a given assembly and verify process consistency for repeatable results. Without verifying consistency, the assembly or PCB can miss reliability targets. And poor thermal profiles risk poor solder joints and damaged components.

Thermal profiling has four steps. Heating brings the assembly to a preheat temperature that evaporates solder-paste solvents. The temperature is raised further so flux in the paste turns from solid to liquid and becomes active. For the flux to be effective, it must remain within a specific temperature range for a certain period. During this period or soak time, board temperatures equalize. It's important the soak time isn't extended or the board gets too hot and the flux is used prematurely.

The solder alloy or eutectic quickly turns from solid to liquid at 183°C. Temperatures are then raised above the solder's melting point for 30 to 90 sec. But to protect temperature-sensitive components the temperature must not exceed a maximum value nor rise too fast.

During thermal profiling, different reflow stages call for different temperatures because they directly affect solder-joint quality. Excess heat at any stage can damage components. This becomes more complicated when lead-free or hybrid assemblies are involved because their thermal requirements differ. Component specifications must be carefully observed during temperature changes.

Cooling, the fourth step, also requires controlled temperature changes. Solder joints formed between components and SMT pads are not optimal when PCBs are IM properly cooled. This creates the possibility of defects surfacing later.

A final stage

A final stretch of testing cycles checks for reliability with four different tests: Bare board, flying probe, ICT (in-circuit tests), and functional. The latter three are most important. Testing varies depending on complexity, OEM investment, and types of results needed from certain testing.

Initially, a bare PCB is tested using IPC356 netlist, a verification tool that assures PCBs are manufactured to OEM specifications and are free of manufacturing defects. However, more rigorous testing is applied post-assembly with later tests.

Flying probe, for example, is best for low volume, highly complex assemblies. It is easy to set up, conduct, and check for open/short circuits and wrong values. This test also verifies component placement and identifies missing components. However, it doesn't perform power-up testing or check for functional failures.

ICT testing is the most tedious, cumbersome, and expensive. Creating an ICT fixture costs from $10,000 to $50,000 and takes four to six weeks to build. However, ICT is ideal for mature products requiring high volume production. It runs the power signal to check voltage levels and resistance at different nodes on the board. ICT is excellent at detecting parametric failures (incorrect voltage outputs, for example), design-related faults, and component failures.

Functional testing verifies board operation and behavior. The PCB is subjected to a sequence of signals and supply voltages. Responses are monitored at specific points to ensure the board operates correctly. An engineer usually specs the test and the OEM defines test procedures. This test is best at detecting wrong component values, and functional and parametric failures.

If a PCB project is limited to a prototype, the OEM will probably not want to pay the prices for ICT. Instead, it will likely rely on a basic flying probe or power-up test. However, if a particular PCB is mature and designed into an expensive, highly reliable system, the OEM will probably pay for the ICT. Each testing stage finds failures, but boards that survive the final test are sure to work properly.

What ISO and FDA approvals really mean

PCB reliability gets an added boost when the electronic manufacturing system provider is ISO 9001:2000 and ISO 13485 certified, as well as FDA approved. These certifications describe a framework and organization for detailing and documenting every aspect of a product being developed to assure it is designed, fabricated, and assembled to meet specifications and be reliable.