How IPC Class 3 requirements impact the layout and manufacturing of boards in critical applications.
A list of some common fabrication tolerances for Class 3 boards.
The assembly must also accommodate Class 3 design.
IPC Class 3 requirements are used to build today’s most critical electronic applications
We heavily rely on electronics for our day-to-day life, and that reliance is based on the fact that well-designed and manufactured devices are long-lasting and exhibit little-to-no operational issues. This isn’t by chance: extensive testing and guidelines developed over decades have provided clear direction for making boards with high longevity and reliability. There is a financial reality to contend with as well, since the cost-benefit tradeoff of additional manufacturing sophistication may be unfeasible for inexpensive and disposable electronics.
IPC Class 3 electronics are the highest grade of manufacturing reliability. Due to the increased scrutiny and equipment sophistication involved in Class 3 production, the yield falls and costs rise. IPC Class 3 electronics are known as high reliability or harsh environment electronics where acceptable downtime is zero. While this places an extra burden on designers and manufacturers, it is necessary for the operation of critical systems like those found in aerospace or the medical field.
IPC Class and Producibility Comparison
Analogous Producibility Level
General electronics - Consumer products where visual imperfections are less important than functionality.
A: General - The preferred manufacturing sophistication.
Dedicated service electronics - Higher performance and reliability requirements than Class 1. Service interruptions need to be dissuaded, but are allowable.
B: Moderate - The standard manufacturing sophistication.
High-reliability electronics - Continuous performance on demand is a design necessity; downtime is unacceptable.
C: High - The reduced manufacturing sophistication.
The Impact of IPC Class 3 Requirements on DFM
For the designer, Class 3 electronics are both more restrictive and allow more freedom during layout. Generally, features become finer while protective measures become more expansive. Not every Class 3 criterion will differ from those of Class 1/2, but some common points of divergence include:
- Conductor spacing - Unless otherwise specified in a controlled fabrication document, the maximum spacing reduction is 20%. This helps promote signal integrity and reduce in-plane coupling by maintaining expected gaps between conductor features.
- Surface conductor thickness - Class 3 boards require a 20% increase in minimum plating thickness over Class 1/2 boards. A thicker plating can better handle high currents and reduce DC resistance, an important feature for HDI boards to maintain minimum spacing.
- Copper voids - To ensure high reliability, no copper voids are acceptable for Class 3 boards. This helps extend service life by maintaining electrical connectivity and mechanical stability over the minimum number of expected thermal cycles during the service life. For vias and other plated through holes (PTHs), thorough barrel coverage is important, as the mismatch in z-axis CTE with the substrate is a common stressor.
- Final finish - Exposed copper on non-solderable areas is allowable for up to 1% of conductor surfaces. Complete solder mask coverage bolsters reliability in the short- and long-term by reducing oxidation and preventing contamination that can lead to shorts.
- Etchback - After desmearing, connectivity between the vertical via barrel and horizontal inner copper layers will either recede from the edge of the plated through hole or protrude inwards. The former case, known as negative etchback, produces less reliable boards, as the connection only extends from the edge of the barrel to the edge of the inner layer. Class 3 boards allow for a gap of .013 mm/.5 mils from edge to edge or 1.5x that distance from the barrel edge to the full thickness of the internal layer. Positive etchback can form 3-point connections between the inner layers and barrels for greater reliability, and processes that result in positive etchback are more conducive to Class 3 manufacturing despite their additional cost.
- Annular ring - PTHs need a minimum annular ring (the copper surface surrounding the drilled hole) to aid in the solderability of through-hole components and provide minimum coverage to prevent current starvation. External PTHs require a .05 mm/2 mil annular ring with no tangency or breakout and internal pads require only half the listed amount.
Similar to the IPC Classification system, IPC denotes producibility levels for electronic features. Unlike the device as a whole, features can differ in producibility levels to indicate the minimum manufacturing sophistication (equipment, training, technical ability, etc.) required to meet the design intent criteria. Wherever possible, designers and manufacturers should work to relax the necessary producibility levels without infringing on Class 3 requirements.
Assembly Is Also Impacted by Class 3 Standards
Class 3 extends to the assembly of its supporting features as much as the fabrication of the board. Designers might wonder how the assembly can support greater reliability, as the vast majority of components are manufactured and soldered to the board as-is. However, certain component aspects – like weight – come under greater scrutiny for high-reliability concerns.
Standoffs - Components weighing more than 5 grams per lead are more liable to incur failure due to vibrational modes. To stabilize these more massive components, standoffs are necessary and can come in a variety of styles. Standoff feet need to be flush and maintain full contact with the board without interfering with or concealing any conductive pathways (i.e., traces and PTHs).
Cleanliness - Standard passive cooling practices may not be enough for certain applications, and in those instances, additional care needs to be protected against processing solutions that could become entrapped at the conducting interface of heatsinks or additional copper surface area. To prevent this situation, a sealing method must be used to prevent the ingress of corrosive or conductive substances that could inhibit performance and service life.
Mounting - Components may only be mounted freestanding (that is, with no support besides the package leads) at a height of .25 to 2.5mm above the board (surface to surface) when the weight/lead is 3.5 grams or less.
Component lead sockets - For components that are not yet set in the design or are heat/vibration/shock sensitive, component lead sockets may provide the requisite standoff height. Ensure non-noble platings are used to prevent fret corrosion from vibration or temperature cycling.
Cadence ECAD Solutions Can Cover Any Class
IPC Class 3 boards add an extra layer of intricacy to design and manufacture, generally complicating both by demanding greater tolerance and precision. Pre-layout should always begin with communication between the engineers, designers, and manufacturers for realizable design intent, but this is of even greater importance with the heightened IPC requirements. Simulation is also crucial for bolstering the pre-production stage of electronic development with accurate models whose insights can be rapidly folded back into the design. Fortunately, Cadence has comprehensive coverage for PCB Design and Analysis Software that simplifies design workloads and accelerates production schedules. The rules of Class 3 DFM are effortlessly handled with the Constraint Manager of OrCAD PCB Designer for smooth and fast design iterations.
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