Multiple vias daisy-chained together have historically been tested to asses via reliability in an attempt to obtain some sort of statistical significance in via sampling. One problem with a daisy-chain of vias is that depending on design, 70-90+% of the measured resistance comes from the circuit traces connecting the vias together and not from the actual vias.

A typical single plated via has a resistance less than 0.001 Ohms. In a .200 Ohm daisy-chain net, only .020 to .060 Ohms represents plated via resistance. A 10% crack/separation in ALL of the plated vias at the same time, would only result in a 1 to 3% change in daisy-chain resistance and would not trigger a failure event. Even a 30% crack/separation in ALL the plated vias in the daisy-chain at the same time would not exceed the typical 10% resistance change failure criteria for daisy-chain test nets. The assumption that all the plated vias in a daisy-chain will fail at the same rate is highly unlikely and it would take a 95+% crack/separation of one or a few plated vias to register a 10% failure change in the resistance of a typical daisy-chain of plated vias. A 0.001 Ohm change in a .200 Ohm daisy-chain (representing a 50+% crack/separation in 2-3 plated vias), would register as a .5% change in daisy-chain resistance, a value that would be seen as electrical noise or slight shift in temperature of the test chamber and would not even be noted.

Each 1 degree C change in temperature within a test chamber results in ~0.393% change in the electrical resistance of copper. Typical Shock/Cycling Chambers are rated at +/- 1.5C which equates to +/- .6% in electrical resistance variation just due to acceptable temperature variation within the test chamber.

It is clear that daisy-chains of plated vias are only electrically sensitive to the end of via failures and cannot readily determine when plated vias begin their failure process. Daisy-chains certainly have their place in via reliability testing at they can determine when a plated via experiences complete failure, but the testing of single plated vias is the only way to observe cracks/separations in plated vias from their initiation through to failure.

Understanding the mechanics of failure is important to tracing the root cause of their failure. Knowing when your plated vias start to fail is important to reliability assessment, material selection and process control. Being able to see the initiation point of via failure with a Single Via net rather then the end of the via failure with a Daisy-chain can enable a better understanding of differences in the manufacturing process. As Plated Via Structures become more complicated and combine multiple technologies to form increasingly complicated structures, having sensitivity to measure small resistance changes resulting from mechanical changes in the via structure is paramount to understanding the interaction and failure mechanisms between these connected structures.

In 2009, a committee from a German automotive manufacturers group (ZVEI) published a method for testing single vias during dual chamber thermal shock/cycling and much of the automotive industry has since adopted this methodology for via reliability. Microtek Laboratories China has 14 Dual Chamber Thermal Shock/Cycling Testers along with HATS™ Testers doing Single Hole Via Reliability Testing. They have been doing Single Via reliability testing for since 2011, primarily for Automotive Industry requirements.

The Patented* HATS²™ Single Via Coupons contains 7 Single Via Nets. Each of these Single Via Nets can be replaced by a Daisy-chain net. This allows the HATS™ unit to collect electrical resistance data from BOTH Single Via and Daisy-chain test nets for the via structures on the coupons. Designers can include 1 or 2 daisy-chain nets along with 5-6 single via nets on the same HATS²™ Single Via Coupon. This should provide a bridge to historical data and create opportunity to see the difference between single via and daisy-chain net test results.

The HATS²™ Single Via Coupon uses Patented Technology* for Test Coupon designs. In order for a company to use this technology in their test coupon designs, they must sign a licensing agreement with the patent holder. Licensing terms and fees will be determined on a case by case basis, but will have reasonable and nondiscriminatory terms and conditions to applicants desiring to obtain such a license. To inquire about a license to use the Technology of the HATS²™ Single Via Coupon please use the CONTACT form in the Navigation Bar.

Most Daisy-chains have minimal statistical significance to the PCB's they represent. Each Patented* HATS²™ Single Via Coupon contains 7 single via nets. Each of these 7 Single Via nets can be changed to a daisy-chain to allow a mix of single via and daisy-chain net data to be collected. Testing 36 of these coupons in one HATS™ Chamber load, you get a sampling of 252 nets of single vias (or single via & daisy-chain) nets while being able to accurately track the initiation and progression of cracks/separation in each via and correlate to historical information with included daisy-chain nets.

To obtain "significance" from a sampling of vias in a daisy-chain, mathematical models can be used with assumptions made regarding the similarity and variability within the population of vias on the PCB's the daisy-chains represent. As an example, for drilled vias, a "similarly manufactured" population might be made up of 1200 vias drilled by a single drill bit. To obtain a result confidence of 95% with a margin of error at 5%, 125 vias from this 1200 via drill bit population would need to be in the daisy-chain. There are many issues with grouping "similarly manufactured" vias to form a via population from which to calculate a statistically significant sampling size, but however you add it up, the numbers of vias needed in a daisy-chain in order to get statistical significance just keeps going up.

Results from daisy-chains without hundreds of vias in them have little statistical significance to the PCB's they represent and are a statistical "feel good" measures at best and at worst, could be hiding small increases in resistance indicating structural via issues. In order to test the high resistance associated with a large daisy chains, the current flow through the daisy-chain must be reduced and with it accuracy and significant digits of resistance measurement. A 50% crack or separation in a single via would go un-noticed as it would only register a change of <0.0005 Ohms in a daisy chain resistance that might exceed 1 Ohm (.05% change). Even if we had several vias with a 50% crack or separation, we would only see noise in the measurements made.

Daisy-chains certainly have their place in via reliability testing to determine total via failure, but single via testing is the only way to track cracks/separations in vias from their initiation through to failure.

The Patented* HATS²™ Single Via Coupon contains 7 single via nets. Each of these 7 Single Via nets can be designed as a daisy-chain net to allow a mix of single via and daisy-chain data to be collected from each HATS²™ Single Via Coupon. Each single via net typically has a resistance less than 0.001 Ohms. Being able to measure percentage change in this resistance range means making accurate measurements below 0.0001 Ohms, a very challenging requirement which requires both adaptation of the test coupon and measurement system.

Resistance of a single via must be pulled out of the total resistance of the entire test circuit which contains Test Lead Resistance, Resistance due to variation in temperature of the sample, Drift (Meter, Wire, Temperature) and finally the Via's actual resistance.

HATS²™ Technology addresses Test Lead Resistance by using 4-wire Kelvin Techniques which determine the resistance of the test leads in one direction. HATS²™ Technology goes a step further by measuring the test lead Resistance in both directions, leading to a more accurate accounting of the resistance of the test leads.

HATS²™ Technology addresses Drift by using 2 specialized circuits built into the surface of the Patented* HATS²™ Single Via Coupon. Once of these circuits is >0.050 Ohms and one is <0.010 Ohms. By measuring these 2 circuits, a HATS™ Tester updated with HATS²™ Technology can calculate Drift and remove it from measurements of Single Via nets, giving a more accurate reporting of the net's value.