Thin Film vs. Thick Film PCB: Discerning Packages

Key Takeaways

• Thin film and thick film resistors are a common SMD choice; designers can further differentiate between cost and performance.

• While the thickness of the package is the most obvious difference, the manufacturing of the two styles has significant overlap.

• Component selection will also want to incorporate the various ratings that define film resistor capabilities.

Selection between thin film vs. thick film PCCB components often defaults to cost, but performance tradeoffs can also play a role.

There’s a vast difference between the electronic packages most students and engineers use in a lab and what they see when they open their electronic devices (intentionally or otherwise). For reliability and cost reasons, the leaded components gave way to surface mount devices (SMD) owing to cost, space, reliable performance, and better manufacturability. But SMD is a wide classification of components, and further specialization in manufacturing yielded the popular chip package, renowned for its low cost and profile. Engineers may be curious about the difference between thin film vs. thick film PCB components when filling out an assembly or perusing a BOM. While cost is likely the primary driver of selection, it’s valuable to recognize the differences each package type offers.

Manufacturing Challenges and Solutions of Thin and Thick Film PCB Components

Thin Film vs. Thick Film PCB Component Manufacturing

Advances in SMD technology within the last half-century have displaced through-hole as the dominant assembly integration method due to advancements in material sciences and manufacturing techniques. There’s a wide benefit of advantages to this approach that enhances the reliability of both the PCB and PCBA. Today’s portability and form factor concerns led to a divergence in component production methods: thin film and thick film manufacturing. The former, although a refined relic of the 1950s, continues to this day due to the cost and reliability of the components. Thin film manufacturing is the more recent development, and while there are noticeable tolerance improvements in the final product, additional cost for thin film resistors provides a market space for the continued use of thick film components.

The main difference between thick and thin film resistors is – unsurprisingly – their thickness, but manufacturing techniques deviate somewhat as well. Both methods utilize a mix of additive and subtractive processes: the former operates in the sub-millimeter range, while thin-film technologies can deposit atom-thick layers of insulators and conductors for an improved “density” of component performance (e.g., dielectrics for capacitors). A couple of common steps for thin and thick film components include:

• Substrate laser ablation - Production begins by lasering an appropriate substrate to the appropriate thickness, with the added benefit of scalability due to increased processing capabilities. Lasering provides the mechanical shape and features of the components – for example, any unique shapes, holes, etc. As a pre-processing stage of component manufacturing, laser ablation also reduces cost.

• Laser trimming - To ensure specific resistor values, the monolithic substrate, a laser trims and cuts the resistors to a predefined length. Laser trimming incurs some performance degradation by reducing the active area of the resistor, as the surrounding region to the cut diminishes in current carrying capacity. 

Laser trimming is a recent development for thin film resistors, whereas before, photolithograph practices dominated. Photolithography's downside is time, material, and resource-intensive as an additive-then-subtractive method, leading to increased consumer costs. Furthermore, traditional photolithography and firing are at a severe disadvantage regarding tolerancing – arguably the primary benefit of thin over thick film resistors.

Additional Design Discernments To Aid Selection

It’s easiest to understand the motivation and expectations of board designers when incorporating these devices by following the framework for device operation. The three main constraints on the manufacturing and assembly are as follows:

1. Maximum continuous power dissipation (alternatively, the power rating) expresses the power limits for a given ambient temperature. As the primary method of thermal routing is through conductive surfaces, this emphasizes thermal board design more than through-hole packaging. Designers must provide ample copper (large area pours and wide traces) and sufficient vias (a good rule of thumb is one via to every .5A, but confirm with thermal simulation and prototype models) to keep devices cool. Intermittently exceeding the power rating is acceptable over short time frames, but continuous overload stresses degrade and eventually damage the device due to heat-related failure modes and aging.

2. Limiting element voltage (LEV), the maximum continuous operating voltage, or voltage rating (all used synonymously) indicates the maximum voltage above a critical resistance value. Below this critical resistance and operating at max power, the voltage increases asymptotically to its limit; beyond the critical resistance, the device reaches the LEV while power drops asymptotically to zero as resistance increases. Typically, exceeding the LEV results in a permanent drift in the sheet resistance, but other performance characteristics are undisturbed.

3. Energy rating represents the resistors’ capacity to withstand power over a single pulse or continuous pulses at the power rating.

For most of these issues, designers may consider using multiple chips instead of a single chip, but design constraints may make this solution impractical. High-rating thin film resistors can ease some design restrictions in situations where:

• PCB flexing/temperature cycling where larger packages are more prone to solder reliability issues.

• Meeting pulse conditions is a design requirement.

• There’s a need to reduce the device weight or total number of discrete components.

Cadence Solutions for Assembly Choices

Thin film vs. thick film PCB component selection needs to balance multiple assembly factors; there is no universally “preferred” package type, as both fit particular design niches. Depending on the tradeoff between cost and performance, thin film or thick film resistors may make more sense for a design, but a design can just as easily include both. To better discern between these packages, robust pre-production simulation can save prototyping time and money: Cadence’s PCB Design and Analysis Software suite gives design teams a comprehensive ECAD environment to accelerate production schedules. Simulation results can seamlessly integrate into OrCAD PCB Designer for rapid PCB layout.

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