1) How do I get started with the IST Test Service?
2) How many test vehicles should be submitted for IST testing?
3) How much will it cost to use the IST Test Service?
4) How long will it take to complete the IST testing?
5) Can I include the conditions/stresses my products experience in the component assembly environment?
6) Can I use the same test vehicle for all levels of technology?
7) Will IST eliminate the need for microsectioning?
8) How often do I need to complete IST testing?
9) What is the minimum number of cycles required to pass IST testing?
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How do I get started with the IST Test Service?
The first component of these 2 questions is usually more difficult and/or 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 as followss:
Determine the performance variability of our PWB vendor base, for product X.
Quantify/Measure the impact of PTH reliability following 1x, 3x, 6x and 9x SMT reflows.
Compare the performance of 2 plating chemistries/3 materials/3 hole sizes/3 constructions, etc.
Measure the process variability of product X over a 3-month period.
Establish a performance baseline for prototype products, before going into production.
Determine Vendor A, B, & C’s capability to produce high aspect ratio holes and/or microvias.
Each of the above scenarios requires further consideration relative to what level of interconnect will be measured during the testing. If the reliability of the Plated Through Hole (PTH) barrel (usually high aspect ratio) is the dominant concern, the test vehicle will be 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, due to concerns related to the possible cracking of thin copper foils or post separation caused by poor metallization processing conditions, it requires that the test vehicle also include one of the products 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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How many test vehicles should be submitted for IST testing?
Customer A is buying their PWB’s from their preferred PWB Supplier. They want to baseline their “wide range” of products type, to achieve this they have narrowed their technologies down to 3 thickness’ levels (.062”, .093” & .125”). In order to avoid a “snapshot” result the products would be produced in 3 separate lots over a 6 week period. The products contain 3 critical hole sizes (.012”, .024” & .045”), they want reliability data on PTH barrel and inner layer to PTH barrel integrity. To increase their confidence in their vendors, the customer has decided to test the products both as received, after 3 SMT reflows and following 6 SMT reflows (possible rework).
The potential increase in test quantities would accumulate as follows:
3 thickness levels x 3 hole sizes x 3 conditions = 27 variables x 32 samples = 864 coupons.
Using a nominal rate of $70.00 to test each coupon, the cost of this project would be $60K+.
In order to bring the cost of testing into a more reasonable budget, the number of coupons per variable can be reduced. As the number of samples are reduced the statistical confidence will also decrease, but because the differences between the variables are small this should not be a limiting factor. The potential cost savings from reducing the sample quantities would equate as follows:
27 variables x 6 samples = 162 coupons @ $70.00 per/cpn = $12K
27 variables x 5 samples = 135 coupons @ $70.00 per/cpn = $10K
27 variables x 4 samples = 108 coupons @ $70.00 per/cpn = $8K
27 variables x 3 samples = 81 coupons @ $75.00 per/cpn = $6K
An important consideration related to reducing quantities are 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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How much will it cost to use the IST Test Service?
Coupon Inspection and Electrical Prescreen ($250.00 for all coupons received)
Upon completion of the IST coupon fabrication, a number of individual coupons or test panels will be delivered to the test service facility. PWB Interconnect will electrically prescreen and measure 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 will document the findings and notify the customer with the prescreening results, including a recommendation related to which coupons should go forward for IST Testing.
Coupon Testing ($75.00 per coupon, up to 500 cycles - ($150.00 per coupon, up to 1500 cycles
After satisfactorily passing through the electrical prescreen activities, and receiving agreement from the customer to proceed, PWB Interconnect will begin the IST testing on the specified quantity of IST coupons. The coupons shall be 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 shall be continuously and simultaneously monitored. Upon completion of the testing PWB Interconnect shall provide 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.
Failure Location via I/R Photo Imagery ($250.00 for all coupons that reached the failure criteria)
After the completion of testing, PWB Interconnect will locate and identify each failure site in each coupon, usiung the Thermographic/Infra Red Photo Imagery process. Dependent on how the coupons performed through IST testing a low number of microsections should be removed and examined to confirm the interconnect failure mechanism(s).
Failure Analysis Services ($85.00 per section - $130.00 per hour for Technical Report Writing)
Upon completion of IST testing and failure location, PWB Interconnect will perform adequate failure analysis to confirm the IST findings. Completing the necessary micro-sectioning analysis on a TBD (to be mutually 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 2 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 as follows:
Electrical Prescreen = $ 250.00 IST Test 32 cpns @ $75.00 = $2,400.00 Failure Location = $ 250.00 Microsection (4 @ $85.00) = $ 340.00 Additional Report Writing = $ 260.00 Total = $3,500.00 -
How long will it take to complete the IST testing?
· How many coupons are submitted
· How long the coupons are required to stay on the machine(s)
(Eg. 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.
The IST system permits a maximum of 6 coupons per test run. The coupons complete a maximum of 288 cycles per day (3 minutes heating + 2 minutes cooling per cycle). The standard testing protocol of 500 cycles requires 2 days on the system, the extended testing of 1500 cycles requires 6 days.
Based on the following criteria; a sample size of 32 coupons (6 test runs), capacity permitted 2, 4 or 6 systems available, testing was spec’ed to a maximum of either 500 cycles (standard) or 1500 cycles (extended), the timing would be as follows:
500 Cycle Test 1500 Cycle Test Electrical Prescreen = 1 Day = 1 Day IST Test 32 cpns on 2 M/C’s = 6 Days = 18 Days Failure Location = 1 Day = 1 Day Microsection/Photo-Imaging = 1 Day = 1 Day Additional Report Writing = 2 Days = 2 Days Total = 11 Days = 23 Days 500 Cycle Test 1500 Cycle Test Electrical Prescreen = 1 Day = 1 Day IST Test 32 cpns on 4 M/C’s = 4 Days = 12 Days Failure Location = 1 Day = 1 Day Microsection/Photo-Imaging = 1 Day = 1 Day Additional Report Writing = 2 Days = 2 Days Total = 9 Days = 17 Days 500 Cycle Test 1500 Cycle Test Electrical Prescreen = 1 Day = 1 Day IST Test 32 cpns on 6 M/C’s = 2 Days = 6 Days Failure Location = 1 Day = 1 Day Microsection/Photo-Imaging = 1 Day = 1 Day Additional Report Writing = 2 Days = 2 Days Total = 7 Days = 11 Days -
Can I include the conditions/stresses my products experience in the component assembly environment?
An increasing number of customers are becoming very 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 undergoing continuous change 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 is an excellent tool that 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, in order to replicate the absolute conditions experienced during assembly.
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Can I use the same test vehicle for all levels of technology?
The extensive 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 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 design or real estate constraints), a pair of coupons will be released to manufacturing.
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Will IST eliminate the need for microsectioning?
After the completion of testing, thermographic/infra red photo-imagery is used to determine the location of each 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 only is required to be 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 minmum number of microsections will deliver the maximum level of information about product performance, failure mechanisms and PWB manufacturing conditions.
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How often do I need to complete IST testing?
PWB Manufacturers
Activity
Frequency
New Technology Introduction
During development & Pre-production phase
Product Baselining
During initial introduction phase
Process Monitoring
Ongoing following baselining activities
Chemical/Material Characterization
As Required
Process Troubleshooting
As Required
Correlation Studies
During initial development phase
Customer Assurance
As Required
Product Prescreening
Prior to long term (air to air) testing
Component Assembly/Contract ManufacturersActivity
Frequency
Impact of assembly/rework stresses
Ongoing following baselining activities
PWB vendor base capability studies
During initial pre-production phase
Process/Product Troubleshooting
As Required
New Technology/Process Introduction
During development & Pre-production phase
OEM’s or End Use CustomersActivity
Frequency
Technology/Design change impact studies
During initial pre-production phase
Product Troubleshooting
As Required
PWB vendor base capability studies
During development & Pre-production phase
PWB vendor base qualification
As per the supplier procurement specification
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What is the minimum number of cycles required to pass IST testing?
The minimum number of IST cycles required should be 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 of the & humidity that will be 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, this permits 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 falls 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
1) Early fallout (infant mortality).
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.
2) Long-term wear out (fatigue).
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. This can results 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 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.