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Laser Direct Testing of IC Package Substrates
October 2, 2007 |Estimated reading time: 5 minutes
As interconnection densities of PCBs keep increasing, so does the cost of advanced electrical testing. Today, investing in additional test capacity has become one of the most strategic decisions for high-end PCB manufacturers. And, even more challenging is in the area of IC Package substrates, with the advent of Flip Chip (FC-BGA) and System in Package (SiP) technologies.
This paper is introducing a new generation of Electrical Test Equipment dedicated to IC Packages.
Laser Direct Testing technology (LDT technology) converts UV light into electricity enabling electrons to move in and out of the conductive traces of PCBs without any contact and therefore permitting the non-contact, "bed of nails-free" testing of complex substrates for Opens and Shorts.
The question this paper would like to address is very straight forward: This, being the testing of FC-BGA advanced substrates, of course.Indeed, the buzz in the PCB industry around the advent of a full non-contact Electrical Test technology has been circulating for quite some time now.
In the past, as President of the Bare Board Test Work Group (BBT Work Group), I have audited many advanced technologies such as e-beam, plasma, and other promising technologies.
Despite the impressive efforts that have been made by great and very knowledgeable personalities, none of them were fully addressing the PCB industry needs for testing advanced substrates ; and none were commercialised.
What are these needs? They are described in the 4 F's :
* Fine Pitch capability* Fast, if possible a few hundreds FC-BGA substrates per hour* Flexible, meaning short setup time* Fixtureless, meaning Bed of nails free or jig-freeBed of Nails Test Systems
Bed of nails Test Systems mainly address one F out of 4: Fast.
Their fixtures are too expensive to be used for testing PCB, except for very High Volume Manufacturing.
Bed of nails fixture for mainstream PCBPCBs like FC-BGA package substrates suffer from this high test fixture cost where they typically range between $30k to more than $100k per unit.
When a single FC-BGA substrate costs a few dollars, its production needs to exceed a few hundreds of thousands to make its testing financially sound.
This scenario might be common for PC CPU applications, but has became a real problem for all other applications (ASICs ...), where production lies between a few thousand and several thousands of pieces per design or part number.Flying Probes Test Systems
On the other hand, Flying probes Test Systems address the complementary three Fs (out of 4) : * Fine Pitch* Flexible* FixturelessUnfortunately, they are too slow for testing large to medium size runs (over a few thousands pieces), their hourly operating cost being too high, simply because they are 30 to 100 times slower than Bed of Nails Test Systems.
FC-BGA substrates
3 billion of the most complex PCBs in the industry, e.g. IC package substrates, will be manufactured in 2008, mainly in 4 countries : Taiwan, Japan, China and South Korea.
The USA also seems to be embarking into this new trend with several important initiatives.
When talking about testing the next generation of IC Package substrates, we have to take into account two additional important needs:* The first one being extremely dense pitches: the capability of testing high density Flip-Chip arrays with pitch going down to 110 um FC-BGA Flip-chip array* The second being the need to test these complex substrates without DAMAGING the above C4 bumps.
More than 2/3 of them will be produced by run sizes ranging from a few thousand to a few hundreds of thousand pieces, and will endure increasing test costs and demands for fast integration time.How can Pr. Einstein help to solve this problem ?
Pr. Einstein said: "Nothing is faster than light".
Moving light
From a physical point of view, moving light with high accuracy obviously appears to be much faster and easier than moving a mass (like flying probers): when you think about it, what is lighter than light ?
Pr. Einstein had another great contribution for Humankind (and incidentally for the PCB industry) : he was awarded the Nobel Prize for his discovery of the photo-electric effect.
The photo-electric effect is the main physical phenomenon used in Laser Direct Testing technology. When a high energy photon meets an electron in a metal pad, the electrons become excited and can escape with residual kinetic energy. Electron movement means electrical current generation.
In this fashion, Beamind converts UV light from laser beams into electrical current in a full non-contact way to test IC Package substrates and other complex HDI PCBs for Opens & Shorts.
We can make electrons break away from trace-ends, but also return into trace-ends. We therefore can create an electrical current going through traces, allowing detection of opens.
Open detection with LDT technologyWe apply a similar principle for the detection of shorts.How does it work ?Laser beams are deflected to hit trace pads by high precision galvanometer-mirrors, such as the ones used for Laser Drilling applications. Photons hit electrons which escape traces and are captured by a plate referred to as "the Collector".Double-sided non-contact access of FC-BGA substratesLaser Direct Testing Systems are capable, without contact as in Bed of Nails or Flying probe test systems, of fully accessing all testable trace pads of Flip-Chip BGA Substrates, including C4-to-C4 Nets.
Several strategic core components of our Laser Direct Testing System were developed in close partnership with the CEA/Leti (www-leti.cea.fr/ & www.minatec.com) in France, near Grenoble, where we have our main facility.CEA/Leti - MINATEC : European "premiere" R&D platform for the semiconductor industryConclusion
By introducing Laser Direct Testing, Beamind's mission is to provide an innovative technological breakthrough to solve many challenges revealed by testing today's IC package substrates, to empower modern PCB manufacturers to electrically test the next generation of High Density PCBs, lowering the global test cost and increasing flexibility on the factory floor.
Just as Laser Drilling and Laser Direct Imaging (LDI) have revolutionised their industry segments, we are entering a new electrical testing era with Laser Direct Testing.
It will change the way IC Packages are produced, paving the way for technological innovations in the integration of FC-BGA package substrates and System-In-Packages.