Frequently Asked Questions

How do I get started with the IST Test Service?
How many test vehicles should be submitted for IST testing?
How much will it cost to use the IST Test Service?
Can I include the conditions/stresses my products experience in the component assembly?
Can I use the same test vehicle for all levels of technology?
Will IST eliminate the need for microsectioning?
How often do I need to complete IST testing?
What is the minimum number of cycles required to pass IST testing?
What is the current software revision for the IST tester?

To determine how PWB Interconnect Solutions Inc. can best support your needs we require answers to two fundamental questions: What are you specifically trying to prove and what level of technology and type of test vehicle is required to achieve it.

The first component of these two questions is usually more difficult and more complex than initially realized. Each customer contacts us for different reasons and has diverse perspectives on exactly what they want to measure, compare, quantify, or confirm. The first step to understanding the scope and magnitude of this effort requires the discipline to draft a mission statement or a clear objective. Examples of possible scopes of work are:


- Determining the performance variability of our PWB vendor base, for the product ‘x’
- Quantifying/measuring the impact of Plated-Through Hole (PTH) reliability following 1x, 3x, 6x and 9x SMT reflows
- Comparing the performance of two plating chemistries/three materials/three hole sizes/three constructions, etc.
- Measuring the process variability of product ‘x’ over a 3-month period.
- Establishing a performance baseline for prototype products, before going into production
- Determining vendors’ capability to produce high aspect ratio holes and/or microvias.

Each of the above scenarios requires further consideration to what level of interconnect will be measured during the testing. If the reliability of the PTH barrel (usually high aspect ratio) is the dominant concern, the test vehicle is designed with a relatively high volume (300 to 700) of small vias on a small grid (1mm or .050”).

If the inner layer to PTH barrel interconnect is to be added, concerns related to the possible cracking of thin copper foils or post separation caused by poor metallization processing conditions requires that the test vehicle also include one of the product’s larger hole diameters. It is recommended that a component hole size (usually a connector hole size) be combined with the small diameter holes. This addition usually requires that the grid size be doubled, resulting in a 50% reduction in hole quantities and the division of the hole quantity between the two levels of interconnect.

Different coupon design rules are considered when additional levels of interconnect are to be added. There are numerous possible combinations, the most commonly used technologies are:

- Plated Through Vias (testing both PTH barrel reliability and inner layer to PTH barrel integrity)
- Blind Vias (most commonly used in combination with sequential lamination)
- Buried Vias (multiple levels throughout the stack-up)
- Micro Vias (multiple sizes and recently multiple levels)

Most “multi-technology” IST test coupons will combine the different levels into as single coupon, with individual connectors responsible for measuring specifically designed circuits. If a single coupon is not possible (due to real estate constraints), multiple coupons or a test panel will be released to PWB manufacturing.

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The preferred number of coupons required to permit a statistically valid (99% confidence level) result is generally 32 samples/per
variable. This number is realistic if the sole intention is to establish a performance baseline for one particular product, with one
set of process conditions, material, or design attributes. The number of samples starts to become prohibitive (expensive) when a number of conditions or variables are added in a design of experiments (DOE).

An example of how the quantities can escalate:

- Customer A is buying their PWBs from their preferred PWB Supplier to baseline their “wide range” of products type.
- To achieve this they have narrowed their technologies down to 3 thickness levels (.062”, .093” and .125”).
- To avoid a “snapshot” result, the products would be produced in three separate lots over a six-week period.
- The products contain 3 critical hole sizes (.012”, .024” and .045”)
- They want reliability data on PTH barrel and inner layer to PTH barrel integrity.
- To increase their confidence in their vendors, they will test as received, after 3 SMT reflows, and following 6 SMT reflows (possible rework).

The potential increase in test quantities would accumulate as follows:

- Three thickness levels x 3 hole sizes x 3 conditions = 27 variables x 32 samples = 864 coupons
- Using a nominal rate of $xx to test each coupon, the cost of this project would be $xxK+

To bring down the cost of testing, the number of coupons per variable can be reduced. As the number of samples is reduced, the statistical
confidence will also decrease – this should not be a limiting factor since the differences between the variables are small. The potential
cost savings from reducing the sample quantities would be:

- 27 variables x 6 samples = 162 coupons @ $xx.00 per/cpn = $xK
- 27 variables x 5 samples = 135 coupons @ $xx.00 per/cpn = $xK
- 27 variables x 4 samples = 108 coupons @ $xx.00 per/cpn = $xK
- 27 variables x 3 samples = 81 coupons @ $xx.00 per/cpn = $xK

An important consideration when reducing quantities is the processing variations associated to electrolytic copper plating quality and/or
thickness distributions; also random defects, like voiding, can create difficulties with the statistical analysis. When the sample sizes
are too small, the results of one or two samples can have a very significant impact – when calculating the mean, outlying data points
will distort the results.

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There are four basic cost components involved with the test service:

Upon completion of the IST coupon fabrication, a number of individual coupons or test panels are delivered to the test service facility. PWB Interconnect electrically prescreens and measures all coupons to verify coupon design and manufacturing consistency. The largest variables are generally related to Electrolytic copper plating thickness and distribution, additionally, drilled hole to internal layer misregistration is a common problem. PWB Interconnect documents the findings and notifies the customer with the prescreening results, including a recommendation related to which coupons should go forward for IST Testing.

After satisfactorily passing through the electrical prescreen activities and receiving agreement from the customer to proceed, PWB Interconnect begins the IST testing on the specified quantity of IST coupons. The coupons are tested until the inception of failure, generally set at a 10% increase in elevated resistance. The resistance through the PTH Barrel and the resistance through the inner layer to PTH barrel (or buried via, blind via, micro via) interconnect are continuously and simultaneously monitored. Once testing is completed, PWB Interconnect provides a report which identifies the test results with all applicable graphs for each coupon. The results also include comparison graphs of the products performance to established baselines for products with similar technology and attributes.

On test completion, PWB Interconnect locates and identifies each failure site in each coupon, using the Thermographic/Infra Red Photo Imagery process. Depending on how the coupons perform through IST testing, a low number of microsections are removed and examined to confirm the interconnect failure mechanism(s).

Upon completion of IST testing and failure location, PWB Interconnect performs adequate failure analysis to confirm the IST findings. The necessary microsectioning analysis on a TBD (agreed upon with the customer), sampling of the coupons failure sites to visually assess the failure locations and modes. A final report documenting the findings of the failure analyses activities shall be prepared with microscopic images. Upon completion of the Failure Analyses activities, the analyzed specimens and remaining IST coupons shall be labeled, packaged and returned to the customer.

The overall cost of IST testing is very much determined by two factors: how many coupons are submitted to IST testing and how long they are required to stay on the machine(s). Based on a sample size of 32 coupons, tested to a maximum of 500 cycles (standard) the cost would be:

How long will it take to complete the IST testing?

The overall time to complete the IST testing is generally determined by four factors:

• How many coupons are submitted
• How long the coupons are required to stay on the machine(s) (E.g. Test to 500 or 1500 cycles)
• How many machines are available to support the testing requirements (Maximum of 12 possible)
• The level/magnitude of failure analysis and report writing requested

An increasing number of customers are interested in knowing whether their PWB substrates are detrimentally impacted by the multiple thermal excursions experienced during the component assembly operation. The assembly environment is continually changing because the designers want to utilize high density interconnect (HDI) and introduce BGA or Micro-BGA into their products. Feature sizes are continually shrinking, constructions are changing to support electrical, thermal and I/O routing needs.

The industry has minimal information/data that quantifies the ability of the interconnects to withstand the exposure to the increasing number of high temperature thermal cycles. The expectation is that this situation will become more critical with the advent of lead-free solders (higher eutectics) and the present issues associated to reworking of BGA and Micro-BGA devices.

IST Technology permits the quantification of the relative performance impact of the assembly and rework environments. The standard approach requires that sufficient test vehicles be produced to initially measure the as-received quality, followed by products exposed to the assembly process. The product’s temperature profile and required number of “thermal cycles” are usually specified by the customer. It is recommended that the initial studies use the customer’s equipment to replicate the absolute conditions experienced during assembly.

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The most commonly used IST test coupon designs are usually configured to measure both the performance of the Plated-Through Hole (PTH) barrel and the inner layer to PTH barrel interconnects. Different configurations would be required if alternative interconnects are added into the IST testing.

The IST design library contains many possible options/combinations, the interconnect types are:

    - Plated Through Vias (TV) - measures PTH barrel and inner layer to PTH barrel integrity
    - Blind Vias (BV) - measures blind via barrel and inner layer to blind via barrel integrity
    - Buried Vias (BV) - measures blind via barrel at multiple levels throughout the stack-up
    - Sequential Lam (SL) - measures core via barrel, PTH barrel and inner layer to PTH barrel integrity
    - Micro Vias (MV) - measures micro via, PTH barrel and inner layer to PTH barrel integrity
    - Step & Repeat (SR) – multiple coupons of same technology with different design features
    - Test Panel (PNL) – complete test panel design with various quantities of different test vehicles

Most “multi-technology” IST test coupons combine the different levels into a single coupon, with individual connectors responsible for measuring specifically designed circuits. If a single coupon is not possible (due to design or real estate constraints), a pair of coupons is released to manufacturing.

No, but it will definitely reduce the required quantities and improve the effectiveness of the microsections that are produced. IST testing starts with a coupon of “good” quality, following multiple thermal cycles the stresses precipitate micro-cracks that eventually coalesce into barrel cracks, foil cracks, or interconnect separations. The system identifies when (how many cycles) and which level of interconnect(s) has failed.

At testing completion, thermographic/infrared photo-imagery determines the location of the failure site(s) in each coupon. The IST data is referenced to establish whether all coupons failed for traditional barrel fatigue or a different failure mechanism. If all coupons performed in a similar manner, a single sample is microsectioned. The section is used to confirm the failure mechanism reported by IST, and ensure the physical features meet the manufacturing specification.

When coupons achieve results below the expected performance baseline, or fail for non-traditional failure mechanisms, additional microsections are usually completed to help diagnose the root cause. The advantage of the IST approach is that a minimum number of microsections will deliver the maximum level of information about product performance, failure mechanisms, and PWB manufacturing conditions.

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The answer is very much related to who asks the question. There are typically three levels of customers that require IST test data;

• 1) PWB Manufacturers
• 2) Component Assembly/Contract Manufacturers
• 3) OEMs or End-Use Customers

The activities requiring IST data for each customer level is as follows:

Component Assembly/Contract Manufacturers

OEM’s or End Use Customers

This is related to the end-use operating environment that the product is designed to experience. The conditions of this end-use environment are usually measured by a combination of the anticipated temperature extremes and humidity experienced during the expected life-span of the product.

An important consideration is whether the reliability assessment is measured in the as-manufactured condition or tested following the multiple thermal excursions applied during the component assembly and potential rework operations. The work completed by PWB Interconnect includes testing of products from both conditions, permitting a quantification of the products ability to withstand the high levels of stress and strains associated with the standard manufacturing environments.

The levels of interconnect performance generally fall into two categories: Early Fallout is caused by defective manufacturing conditions or excessive stresses applied during assembly or rework; Long-Term Wear Out is precipitated by the product’s inability to withstand the end-use environment.

Premature plated-through hole barrel cracks or inner layer to PTH barrel to copper foil fractures are characterized by rapid crack propagation with low-energy release and without significant plastic deformation.

Fatigue is caused by repeated cycling of the load. It is a progressive localized damage due to fluctuating stresses and strains on the material. Fatigue cracks initiate and propagate in regions where the strain is most severe. The process of fatigue consists of three stages:

    - Initial crack initiation
    - Progressive crack growth around or across the structure
    - Final sudden fracture of the remaining cross section

Fatigue is a significant problem because it can occur due to repeated loads below the static yield strength resulting in an unexpected and catastrophic failure in use. Because most engineering materials contain discontinuities, most fatigue cracks initiate from discontinuities in highly stressed regions of the interconnect. The failure may be due to the improper manufacturing conditions, design, poor material selection, or other causes. IST testing, in combination with failure analysis, can uncover the defect and assist in determining the root cause of the failure mechanism.

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