Film clarifies electronics packaging problems

Analysis of contact uniformity in PCB assemblies and verifying planarity or flatness between semiconductor wafers and bonding heads was, until recently, a largely inaccurate and unscientific process.

Analysis of contact uniformity in printed circuit board (PCB) assemblies and verifying planarity or flatness between semiconductor wafers and bonding heads was, until recently, a largely inaccurate and unscientific process.

Engineers were limited to using carbon paper or conventional pressure measurement tools such as load cells and strain gauges (often too invasive or time/cost intensive).

Today, tactile pressure sensor film (TPSF) is an economical metrology tool for successfully predicting and evaluating compressive load distribution.

Use of TPSF technology can reveal problems with design or production of PCB assemblies, such as overloading of connector contacts, standoffs, and component solder joints, and semiconductor wafers, ie improper lamination press planarity caused by inconsistent bonding pressures.

It can also confirm computer modelling predictions generated by such preprocess tools as finite element analysis (FEA).

TechPro Engineering, located in Brussels, Belgium produces PCB assemblies.

The company needed a quick means of evaluating pressures on PCB assembly mounting points or connectors.

TechPro's Lead Engineer, Peter Verboven says: "We were concerned about the magnitudes of pressures being exerted on the PCB connector contacts".

"We were able to use the tactile pressure film as the initial-evaluation method to determine this information".

Osram Opto Semiconductors, located in Regensburg, Germany manufactures semiconductor wafers.

The firm wanted to verify lamination press flatness between wafers during the bonding process.

Osram Opto's Engineer Marianne Renner says: "We could not directly measure the planarity between wafers during the actual bonding process; we could only do so prior to bonding".

"Even though we ran the tests at room temperature rather than at the actual working temperature, the film was able to discern planarity problems".

"The film allowed us to audit the predicated bonding pressures and confirm FEA predictions".

A single-use system, TPSF captures a snapshot of maximal pressure loading at a specific moment in time.

It reveals a high-resolution image of how pressure is distributed across the entire surface and its magnitude.

The film is a thin sheet that can range in thickness from 0.1016 to 0.2032mm.

These sheets are structurally affixed to a Mylar substrate for stiffness and are still pliable enough to allow the film to adapt to intricate or curvaceous surfaces.

TPSF types are divided among seven specific parameters, from 2 to 3037kg/cm2, to maximise the accuracy of pressure measuring data.

Selecting the appropriate sensor film type is simple once an engineer calculates approximate amount of pressure loading.

Microcapsules embedded in the film rupture at precise pressure levels causing them to release an activator component, which interacts with a developer to produce a visible colour change.

This quantifiable colour change is directly proportional to the amount of pressure applied.

The spatial resolution of TPSF is 0.005 to 0.015mm, yielding ultrahigh definition of force profile imagery.

The resulting image can then be visually inspected for significant variations, pressure aberrations and general uniformity.

The colour change of the film is both instantaneous and permanent, allowing for the film to be immediately analysed and then archived for future analysis.

Once compressive load has been applied, TPSF can be analysed in several different ways.

The quickest, most economical technique is by visual comparison to a colour calibration chart similar to using litmus paper.

The film's resultant colour can be matched to a colour intensity chart to obtain an approximate indication of pressure magnitude.

This visual interpretation method yields an accuracy rate of +/-15% full-scale (kg/cm2).

Using an optional, enhanced optical imaging tool offers a more sophisticated and accurate technique of analysis and interpretation.

One such optical image analysis system consists of Windows-based software and a specifically calibrated scanner that reads and interprets the TPSF.

Images rendered by this analysis system are accurate to +/-2 to +/-3% full-scale (kg/cm2); interpreted images are accompanied by a wealth of graphical and statistical data about the contact pressure test.

Not only is analysis of the entire interfacial surface possible, but small and problematic areas can also be enlarged and enhanced for careful scrutiny.

As Peter Verboven of TechPro noted regarding pressure magnitudes on the contact points of PCB assemblies, TPSF "verified the expected contact pressures as well as showing us we were exceeding the minimal contact pressure we needed for proper contacts".

"For us, the film confirmed the expected contact pressures set in the contact design".

"We were pleased and we are sure additional analysis using the compatible optical image analysis system would improve reliability of our measurements".

"We believe this tool to be especially attractive in the cases where contact pressures have to be measured in difficult to access or small contact regions".

"Affordability and ease of use of the product was certainly sensible for our stated needs and budget".

Commenting on her need to measure flatness between semiconductor wafers during a bonding process, Marianne Renner at Osram Opto noted: "We successfully used the tactile pressure sensor film as a means of auditing the allocation of lamination press planarity pressures before the wafer bonding process itself".

Employing TPSF has improved quality for hundreds of OEMs interested in different electronics packaging processes.

In conjunction with its optional post-process optical image analysis system, TPSF lends a high degree of statistical validation to contact pressure magnitude analysis for not only printed circuit board contact pressure points but also for wafer bonding uniformity.

Use of this tool can significantly improve both the design and production processes employed by electronics OEMs while at the same time reducing budget over-runs and contributing to leaner, more accurate manufacturing methods.

The TPSF mentioned in this article is specifically known as Pressurex; the optical image analysis system is known as Topaq.

Back to top