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Off-line strategy forms the basis

3D AOI for high-precision control of SMT process parameters
Off-line strategy forms the basis

Approximately 60bn semiconductor chips are presently produced worldwide. All of these ICs are contained in some form of housing. Regardless of whether IC or system level packaging is used. Prof. Tumalla of Georgia Tech in Atlanta/USA contemplates that such packages have proven to be a bottleneck in manufacturing.

Bernd Kagerer, Rainer Brodmann, Marcus Grigat, Nanofocus

Further miniaturization and cost savings are important targets of board assembly. In the course of these developments, the requirements placed on dimensional measuring technology have become higher and more varied. While it was sufficient totolerate warpage of substrates or co-planarity within the range of 100 microns just a few years ago, today the tolerances are just a few microns. Flip chip, chip-on-board (COB) and CSP are just a few topi-cal subjects that require high precisionin board assembly and its connection methods.
Miniaturization in connection technology
With traditional 2D AOI systems, the technological measuring limits are being reached to an increasingly greater extent. The trend is therefore towards measuring systems capable of verifying micro-geometries in all three dimensions (3D AOI). Particularly during the introduction phase of technologies, quick and precise measurement of parameters such as height, distortion or volume is required. By collecting process and material-related experience during the early stage of process development, potential defect sources in a proposed volume manufacturing can be determined. For this application, a new type of laser profilometer that operates with off-line inspection strategy using high-precision sensor technology is an excellent fit for flexible application in product quality control.
High reliability of the individual production steps is the most important requirement in modern component production. In the assembly of CSP components, the contact pitch lies very often under 0.1mm. This means that the solder volume ofthe bumps on the board cannot exceed 100-mil³. Multi-chip modules (MCM) based on HDI (high-density interconnect) boards with conductive holes, buried and blind vias whose diameters are less than 100µm deliver another example. Using these interconnection technologies require a thorough implemen-tation in a stable production process.
Why high-precision 3D inspection?
Typical AOI systems are designed for 100% production control. They have to verify during the process if the quality of the assemblies is sufficient. Cameras and image processing or line scanners based on triangulation technique – all working with high-speed – are the primary elements of these AOI systems. The maximum lateral and vertical resolution is in the range of 10 to 20µm and, in terms of the analysis capabilities, these systems are typically used to perform Go/No-Go decisions. Information regarding the production process by specifying the types of defects can only be found to a limited extent. However, complex manufacturing methods for high-density interconnect assemblies require precisely quantified evaluation for process control.
Since the structures to be measured are becoming increasingly smaller, for reliable information the use of equipment capable of resolutions of less than 1µm in height and 2 to 3µm in horizontal direction is required. This is achievable only with high-precision 3D measuring systems. Here, the focus is not on checking the components, but rather the precise measurement of specified geometric characteristics on random samples. The data obtained then helps to control the production process and to adjust it precisely on the basis of these parameters. Values that are continuously positioned in unwanted ranges are detected by statistical process control (SPC) and allow the process to be corrected on-line. Such a quality-control loop is, for example, the verification of the volume of microscopic drops of glue serving to control a dispensing process.
Laser profilometer as off-line solution
A system capable of satisfying the high requirements of off-line inspection strategy is the µScan Pro of Nanofocus. A high-speed confocal dot sensor with a Z-resolution of less than 0,1µm provides for the required data accuracy. The laser spot is minimized, so that the instrument is well prepared even for upcoming smaller structures. The confocal measuring principle is particularly suitable for steep structures such as bumps, printed solder paste and microvias, because with this technique (physical filtering with pin hole), measuring signals are registered only when the „real surface“ is present in the measuring area. This means that undesired artifacts – as such on the basis of multiple reflections, control signal effects through signal loss and shadow effects with conventional methods (triangulation, stripe projection, autofocus) – are virtually excluded here.
The entire measuring system is extremely robust due to its application on the shop floor, and can also be operated quickly even by untrained staff. Thanks to itsmodern and flexible software concept, it is easy to automate and can be adapted to the specific measuring requirements. The calculation of parameters is accomplished individually and fully automatically. For example, is it possible to control the system, the data capture, the evaluation and even further processing from an Excel application. A link to a SPC system completes the instrument for process monitoring.
How and where can 3D inspection be used?
With high-precision optical 3D inspection in combination with functional test, it is possible to achieve a closed quality-control loop in the production process. This is comprised primarily of two interwoven procedures. The 3D inspection provides data that is analyzed and evaluated statistically. The parameters achieved in this manner canbe used both for adjusting the production process on one hand, and for correlation of results from the functional test on the other. This will improve the knowledge in process development and package design. Optical 3D inspection is accomplished off-line using random samples, while in many cases a functional test is performed to 100%.
From our point of view such an off-line inspection strategy is always advantageous when highly complex assemblies are being produced using advanced processes. The experience obtained from continuous monitoring and adjustment of process parameters combined with the improved knowledge in development and design leads to significantly lower failure rates. What examples are available for successful implementation of this strategy?
Volume control of fine-pitch solder paste
As a rule of thumb, the solder paste-printing step represents the most critical process in the SMT board assembly. About 65% of the defects are related directly or indirectly to this step. Relevant process parameters include the height of the soldered bumps and their volume as well as position directly following stencil print. Particularly fine-pitch paste print and the use of further solder materials require precise measuring methods. Key details of solder paste print can be imaged easily with the instrument.
However, even more important is the fact that the high-precision data allows exact evaluation and definition of the desired process parameters. From the XYZ coordinates which are precise to a tolerance of ±1µm laterally and less than ±0.1µm vertically, the volume, height, co-planarity and gradients can be calculated within fractions of a second. Moreover, the exact position of a solder bump can be determined by means of calculating the point of gravity. This provides a complete and reliable verification of parameters for vari-ous process steps. When used as an off-line inspection unit, random samples are measured from various stages in production. In addition to the easily recognized production difficulties, this also allows the clear localization of the step in a line.
Co-planarity analysis for µBGA
The number of BGA I/Os increases continuously while the package area available per solder ball decreases at the same degree. This trend to µBGA or CSP makes a highly precise 3D control of the ball grid necessary. The height distribution of the solder balls and their position in relation to each other have to be checked particularly. This is easy to accomplish with the help of the laser profilometer. From this data, co-planarity of the individual solder balls in regard to the others can then be computed with the aid of a 3-point contact surface or regressive compensation area. Simultaneously, it is also possible to measure the distortion of the package which allows an assumption in regard to the true ball height. Warpage and co-planarity are relevant parameters for many process steps.
Modern production of SMT board assemblies increasingly requires precise 3D measuring technology for the control of microstructures on board or component, respectively. Off-line inspection allows economic determination of process parameters. In contrast to Go/No-Go decisions, precise 3D measurement allows production processes to be tightly controlled. In addition, it has also proved beneficial to define and standardize values of the individual process steps. Settings and variation limits should each be evaluated. High-precision 3D measuring data has proven extremely valuable for the understanding of the various production steps.
Zusammenfassung
Typischerweise denkt man bei 3D AOI an Produktionssysteme, die in der Linie der fortlaufenden Qualitätsüberwachung aller Baugruppen dienen. Aber zum Einfahren von Prozessen, zur Untersuchung von einzelnen Prozeßschritten und äußerst differenzierter Defektanalyse ist ein spezielles, confokales Lasersystem zur off-line Anwendung entwickelt worden, das Auflösungen der Meßwerte bis herunter auf 0,1 Mikron ermöglicht.
Résumé
Quand on parle de 3D AOI, on pense systématiquement à des systèmes de production qui servent, dans la ligne de fabrication, à surveiller de manière continue la qualité de tous les modules. En revanche, pour la mise au point des processus, l’analyse des différentes étapes et l’analyse très différenciée des défauts, on fait appel à un système laser spécial, développé pour les applications off-line, avec lequel la résolution des valeurs mesurées peut atteindre 0,1 micron.
Sommario
Quando si parla di 3D AOI si pensa solitamente a sistemi di produzione che all’interno delle linee servono per il controllo continuo della qualità di tutti i gruppi costruttivi. Per l’avviamento di processi, per il controllo di singoli fasi di processo e per permettere un’analisi estremamente differenziata dei difetti è stato sviluppato uno speciale sistema laser confocale per uso off-line che permette risoluzioni dei valori di misura sino a 0,1 micrometri.
Current Issue
Titelbild EPP EUROPE Electronics Production and Test 11
Issue
11.2023
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