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Once the family hears you have some background knowledge of electronics, conversations quickly become, &#;Y&#;know, I have this device that went on the fritz recently.&#; Perhaps an evolution of the fledging IT role many computer-savvy teenagers played, those in the industry are aware of how difficult even diagnosing some electronics issues are, much less their repair. 

Sure, a solder job is easy enough with an iron and a steady hand, but malfunctions emanating from the board are nearly impossible to correct. As electronic devices have become more sophisticated, the board itself has become a component in many ways that requires a significant amount of engineering design. To counter this mounting complexity, fabricators use a test coupon to enact quality control across the many manufacturing processes.

The Role And Standards of A Test Coupon

Test coupons are manufacturers&#; primary mechanism to verify PCB production processes. Unlike electrical testing, which typically only measures the performance or function of board features, test coupons allow observers to evaluate form and design. The difference is subtle but essential: function may be intermittent over device operating conditions (sometimes befuddlingly), and mechanical/material characteristics are always at the core of issues. 

The test coupon is a sacrificial piece of a PCB panel explicitly reserved for destructive analysis. Associating the coupon with the panel in this fashion has a higher likelihood of creating a representative testing sample that accurately reflects the manufacturing environment of the boards.

Coupons are valuable as they enable destructive testing without impacting yield, so long as the coupon size doesn&#;t preclude additional boards through panelization, although this is unlikely. Test coupons have two potential drawbacks:

1. The coupon separates from the board(s) it represents. Poor tracking and organization may result in the unavailability of the coupon when further assessment is necessary. &#;

2. The fabricator designs the coupon features, not the person or team performing the PCB layout. Multiple designers at different stages can introduce an additional source of error when test structures do not adhere to the corresponding layer&#;s single-ended or differential pair impedance.

While important, effective communication between design, testing, and fabrication facilities can circumvent these errors with proper documentation practices. Additionally, including the test structures within the board can take up significant real estate &#; impedance test traces can occupy a run distance greater than three inches &#; making them highly unsuitable for HDI designs or small enclosures.

As destructive testing supports multiple methods of manufacturing analysis, IPC has defined conformance parameters for multiple coupon styles within IPC-B. This piece will not dive into the minutiae of the IPC standard but instead provide the reader with an idea of the different analysis modes and their suitability to PCB manufacturing processes.

Coupon Types (as adapted from IPC-B)

Coupon

Test feature(s)

Purpose

AB/R

General through-hole features

Evaluates PTH/vias, thermal stress, clearance, registration

A/R

General through-hole features (no vias)

Evaluates PTH, thermal stress, clearance, registration

B (propagated)

Vias (blind, buried, through, filled, etc.)

Evaluates vias, thermal stress clearance, registration

E

Moisture insulation

Moisture insulation resistance

S

Hole solderability

Evaluates hole solderability

W

Surface mount solderability

Evaluates surface mount solderability

D

Daisy-chain via

Plating/via thermal stress

D (propagated)

Daisy-chain propagated via

Plating/via thermal stress

G

Solder mask 

Solder mask adhesion

H

Surface insulation

Surface insulation resistance

P

Peel strength

Peel strength/plating adhesion

Z

Controlled impedance

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Controlled impedance

K

Registration

Verifies clearance of internal plating/copper features

Additional Detail on IPC Test Coupons

AB/R

A combination of the legacy A, B, and R coupon features, AB/R improves on the three by adding a C feature for the via or component hole with the smallest annular ring. B features check the internal lands on the layers closest to the top and bottom, signal layers, and plane layers. This coupon is still compatible without B features, although the fabricator will need to edit the drill file before and after generating the coupon. R features establish registration, but a variance in foils heavier than 1 oz. precludes their usage. Similar to the AB/R, the A/R coupon is applicable for boards without vias.

B (propagated)

B coupons evaluate variant via structures, such as a combination of (un)filled through-holes or blind/buried microvias, that are outside the scope of the B features of the AB/R coupon. The propagated B coupon aligns with the D coupon structures for feature testing and lacks the registration features of the AB/R, but otherwise, its intent is the same as the AB/R. Each B coupon supports two via structure styles, with the vias further subdivided into an upper and lower half. The top half is used for registration purposes, while the bottom examines structural integrity.

E

These coupons check the moisture/insulation resistance of the laminated substrate material at a maximum of ten layers at a time. Boards with greater than ten layers must have additional coupons of up to ten layers that start on the layer of the last coupon.

S

These coupons evaluate through-hole solderability. While representative copper layers may remain on internal layers, coupons must suppress all internal pads.

W

This coupon uses a double-sided pattern for surface mount solderability evaluation. Neither inner layer patterns nor solder mask is included for this coupon, while copper is added to the inner layers to match the PCB thickness.

D/D (propagated)

The D coupon types evaluate via and general hole plating reliability with thermal stress testing. These coupons use via structures in a daisy-chain formation to measure resistance, with the propagated D coupon evaluating more complex via structures as outlined in the propagated B coupon: (filled) through-holes and blind/buried vias (stacked or staggered) are all valid structures, but each coupon can only test for two structure styles.

G

Measures solder mask adhesion over three regions: copper/surface finish, laminate, and interconnect conductor width. 

H

H coupons quantify the effects of residues on surface insulation resistance with an interstitial comb pattern on each side.

P

P coupons determine the peel strength of foils laminated to the outer layers with a conductor pair on both sides.

Z

Z coupons verify the impedance value of controlled impedance lines of the board. Each layer allows for two single-ended or differential pair impedance structures, with a total capacity for 24 single-ended lines, 12 differential pairs, or a combination of the two structure types.

K

K coupons check for clearance between copper on internal layers and hole plating; tests for shorts between these features using an ohm meter.

Your Contract Manufacturer Knows PCB Design Inside and Out

Test coupons offer tremendous insight and feedback on PCB design and manufacturing processes. Instead of having internal features limited to X-ray inspection techniques, fabricators can quickly assess and correct processes, saving themselves and their clients valuable time and money. 

End-users also benefit from coupons in the inspection process as it enhances reliability and improves overall performance outcomes of the board. PCB manufacturing is an involved process, but the engineers at VSE are committed to building electronics for our customers for a bevy of life-changing and life-saving applications. Alongside our valued manufacturing partners, we&#;ve dedicated our professional experience to improving board quality and customer satisfaction over the past 40 years.

If you are looking for a CM that prides itself on its care and attention to detail to ensure that each PCB assembly is built to the highest standards, look no further than VSE. Contact us today to learn more about partnering with us for your next project.

PCB Test Coupons: Everything You Need to Know

In the intricate world of PCB manufacturing, ensuring the quality and reliability of fabricated boards is of utmost importance. PCBs serve as the foundation for countless electronic devices, ranging from smartphones and computers to medical equipment and aerospace systems. As such, any flaws or defects in the manufacturing process can have significant consequences, impacting functionality, performance, and even safety.

To mitigate these risks and achieve optimal board quality, manufacturers employ various techniques and methodologies. One such technique is the utilization of PCB test coupons. These additional boards, produced alongside the main PCBs, play a vital role in evaluating and validating the manufacturing process.

What Is a PCB Test Coupon?

Test coupons are small sections of the PCB designed with specific dimensions and electrical properties to detect manufacturing defects. These coupons include trace widths, spacing, and impedance values, making them instrumental in quality assurance. Common types of test coupons used in PCB assembly encompass single-ended, differential, and impedance test coupons, each representing different impedance requirements of the PCB.

When it comes to placement, test coupons can either be located on the same PCB panel as the main board, typically at the edges or corners, or integrated directly into the PCB layout. While separate placement minimizes interference with signal traces, integrating test coupons provides a more accurate representation of the manufacturing process and electrical properties. However, this integration can present design challenges and potential interference.

The decision of whether to separate or integrate test coupons depends on factors such as the complexity and size of the PCB, as well as the required level of accuracy in testing.

Scope and Limitations of Test Coupons

Test coupons offer a valuable solution for PCB designs requiring complex structures, high reliability, or high-speed signal traces. Nonetheless, simpler PCB designs may suffice with alternative testing methods like in-circuit and flying probe testing.

Test coupons allow manufacturers to pinpoint defects or issues before final product assembly, minimizing the time and cost associated with debugging and redesigning a PCB. However, it&#;s crucial to recognize that the test environment may not precisely mimic real-world application conditions.

Disparities in scale, exposure, and stress can affect PCB performance in practical scenarios, potentially eluding detection by test coupons alone. Therefore, test coupons should be complemented by other testing approaches to ensure the utmost quality and reliability in PCB design.

What about test coupon use at high frequencies? Working with extremely high frequencies poses challenges, especially when dealing with unique materials or HDI boards. These scenarios often give rise to signal integrity issues that require careful attention. In high-speed digital systems operating in the 10s of GHz range, all test points on a standard coupon become crucial and warrant examination on a high-frequency test coupon.

Furthermore, gathering additional measurements is essential to ensure signal integrity and minimize radiated electromagnetic interference (EMI). By considering these factors, PCB designers can address the complexities associated with high-frequency designs and optimize the performance of their systems.

To Integrate or Not to Integrate?

When deciding whether to integrate or keep test coupons separate, there are key considerations to ponder. In some cases, the designer can integrate the test coupon directly into the actual PCB rather than housing it as a distinct section on the same panel. This integration may result in a test coupon that doesn&#;t resemble the conventional ones provided by vendors.

Although it may seem counterintuitive to allocate PCB space for test structures, integrating them directly onto the board offers significant advantages. This approach greatly facilitates in-circuit testing during both prototyping and full-scale production. Designers working with unconventional interconnect geometries can benefit from assessing impedance before moving to mass production.

By designing a single board with the desired interconnect designs and conducting in-house testing, the upfront investment in a test board pays off. It can potentially save the need for a costly board revision later by obtaining crucial measurements prior to production.

Non-impedance Uses of Test Coupons

Test structures placed on a custom-designed test coupon enable the measurement of conductor losses, interconnect impedance, propagation delay, and PDN capacitance. Additionally, these structures prove invaluable in determining the dielectric constant of the substrate laminate. As the frequency moves into the microwave and mmWave range, it becomes crucial to test for insertion loss and cavity radiation to safeguard against significant degradation of analog signals on controlled impedance lines.

Test coupons offer the opportunity to subject them to a range of tests, including reflow simulations, thermal shock, glass transition temperature measurements, and various other custom tests.

By utilizing test coupons, a PCB assembly company can assess the manufacturing process and ensure the quality of the new board, thereby meeting the required reliability standards. Results from the panels should align within a 5% tolerance of the specified values.

PCB test coupons prove to be a seamless solution for verifying the impedance values of PCBs. With identical layer stack-up and tracks as the PCBs, the holes on the coupons accurately conduct the impedance trace. This convenient setup enables the use of impedance test machines to simply plug the test probe into the coupon holes and verify the impedance value.

Achieving matched impedance values with the track lines is crucial for the proper functioning of electronics after PCB assembly. Reputable PCB manufacturers often provide impedance coupons and their corresponding test reports to buyers, ensuring a thorough quality check without any additional cost.

By leveraging impedance coupons, manufacturers and buyers can confidently validate the impedance performance of PCBs, contributing to the overall reliability and functionality of electronic systems.

Contact us to discuss your requirements of OEM PCBA Service. Our experienced sales team can help you identify the options that best suit your needs.