Testing Local Stress in Glass Fibre Composites used in Printed Circuit Boards using Hot Probe Attach Technique

Glass fibre is a material that consists of numerous extremely fine fibers of glass. Composites made with glass fibres are widely employed in the development of printed circuit boards (PCBs). These materials are known for their excellent electrical properties and also exhibit remarkable strength and rigidity.

Although PCBs do not experience complete mechanical failure, copper bonding pads are subjected to local stresses which can result in tiny cracks in the composite material placed proximal to the bond pad.

These cracks can further extend under cyclic loading and lead to electrical failure of the board. Even when there is no external load, difference in Coefficient of Thermal Expansion (CTE) can cause stress which changes according to the temperature.

In order to assess the suitability of the materials, controlled cyclic loading can be applied to bond pads, and the number of cycles to failure can be determined as a virtue of the applied load. However, the bond pad is relatively flat, which makes it difficult to grip the load.

Hot Probe Attach Technique

Nordson DAGE has developed an advanced technique called Hot Probe Attach (HPA) which employs a patented technology to overcome this issue. In this technique, a small pin is attached to the bond pad and load is applied at different angles. This eliminates the difficulties faced with gripping tiny objects.

Cycles to failure for loads applied at 30° to the pad normal.

Figure 1. Cycles to failure for loads applied at 30° to the pad normal.

Board flexure can create severe stress in BGA bonds, specifically at the corners of the die, with the major stress lying about 30° to the pad normal (Figure 1). HPA is capable of making contact at any angle to replicate true service conditions.

Hot Probe Attach Technique

Figure 2. Paragon™ simplifies the entry of test parameters.

Paragon™ software is an easy-to-use interface that helps in producing temperature profiles and also to set up test parameters according to IPC9708 standards (Figure 2). The 4000Plus test equipment includes optics that allows the pin to be accurately placed on pads measuring down to 100µm.

Method

After the pin is lowered onto the solder pad, it is accurately positioned by means of high precision servo stages. At the beginning of the test, the temperature of the probe is increased to the set reflow temperature and then a hold and cool down cycle is performed.

At the time of the thermal cycle, the modifications in the length of the pin are off-set by the software. After the pin is securely adhered to the bond area, the pin clamping mechanism is activated so that the pin can apply the load.

A choice of test modes are available to user. In case of a simple static test, the load is ramped up to failure as the load-displacement information is being recorded. A more detailed form of loading involves cycling between fixed loads where hold times and ramp rate are set through an intuitive interface.

After a set number of cycles are completed, the test can be stopped. This results in sub critical cracks before the ultimate failure under monotonic loading. A suitable work holder is then used to achieve angled testing which holds the sample at the required test angle.

Results and Discussion

It has been shown that cyclic testing is linked with board level drop testing, while the HPA method provides a fast and economic alternative to this form of testing.

Angled pull tests offer a lower number of cycles to failure since the crack that occurs initially lies normal to the pad, and the load’s angle makes sure that there is a component of the applied load which is normal to this crack.

HPA can be utilized under all conditions where a flat surface has to be held and a load needs to be applied. One typical example is metallic coatings on plastic parts, where adhesion between the part and the coating needs to be evaluated. As such, a range of solders can be employed to provide a trade-off between reflow temperature and joint strength. In particular, low melting point solders are ideal for plastics that have considerably low glass conversion temperatures.

Conclusion

Therefore, the Hot Probe Attach provides a fast and cost-efficient method for testing local stress in glass fibre composites utilized in PCBs.

About Nordson DAGE

Dage was founded in 1961 and is a market leader in its chosen markets of Semiconductor and PCBA Manufacture. It has an award winning portfolio of Bondtester and X-ray Inspection Systems for destructive and non-destructive mechanical testing and inspection of electronic components.

Dage was acquired by the Nordson Corporation in 2006.

Nordson DAGE has a strong portfolio of award winning products for destructive and non-destructive mechanical testing and inspection of electronic components. It has an excellent, wholly owned distribution and support network of seven offices covering Europe, Japan, China, Singapore, and the USA, and maintains representative offices in other territories.

With its self-contained R&D facilities, Nordson DAGE has developed world-leading products for testing wire bonds on semiconductor packages such as BGA, Chip Scale Packages (CSP) and other electronic components. More recently Nordson DAGE has been heavily involved in the testing of the newest technology 300 mm wafer bump shear.

Nordson DAGE has developed an excellent suite of award-winning X-ray inspection equipment targeted at both the Semiconductor and PCBA markets.

Control of the patented core technology of X-ray tube manufacture ensures that the high-resolution X-ray will detect, identify and measure even the smallest of features. Nordson DAGE's high precision, state-of-the-art inspection equipment when joined with their sophisticated software offerings, ensure that the equipment is simple to use.

Nordson DAGE

This information has been sourced, reviewed and adapted from materials provided by Nordson DAGE.

For more information on this source, please visit Nordson DAGE.

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