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There's more to IC design than just circuits

Posted: 01 Apr 2016 ?? ?Print Version ?Bookmark and Share

Keywords:IC packaging? DIP? surface mount? package card?

It takes all engineers on a team to bring an extremely complex and advanced product to market. IC designers shouldn't be so stuck in the circuits that they forget the importance of IC packages.

While most design engineers are using surface mount parts, note that Microchip still sells parts in DIP packages, for the hobby and legacy markets.

While sifting through my desk I came across some physical cards with various IC packages on them. They are rather old, but still a good reference. Here is a card I got from ON Semiconductor:

ON-Semiconductor-IC-packages

Figure 2: Card from ON Semiconductor

It never ceases to amaze me how small electronic parts have gotten. I am old enough to remember when surface mount parts were introduced. Many engineers resisted, especially since the smaller parts are harder to prototype and solder. One issue was precision; the smaller parts put more stress on the die during molding. This meant that many parts had worse specs on the same die in smaller packages. There were also reliability problems where moisture would creep up the pins and cause the dreaded "purple plague," a corrosion of the bond wire and pad:

Purple-plague-IC-corrosion

Figure 3: Purple plague caused corrosion.

Soon most engineers realised that the small packages had more benefits than problems, especially after manufacturers solved the reliability and precision problems with better plastics and advanced molding techniques, combined with post-packaging trimming. After engineers solved the prototyping difficulty, we learned to love the small size. The low mounting height meant boards could nest closer together in complex systems. The surface-mount packages were far more resistant to vibration problems. This got industrial equipment designers as proponents. Once our prototypes went into production, we saw how much tighter we could place components since there were no through-holes that obscured the paths of traces on the backside of the board.

Below is another example package card from MCC. It's good to see companies recognise that the package is sometimes the most critical selection criteria of a systems design engineer. Oftentimes, IC designers so rule semiconductor companies, they think the only important thing is the die, and the different packages that die comes in is nearly irrelevant. IC design has become easier with the great tools from Cadence, Mentor Graphics, Synopsis and other tools. Package design is still wickedly complex and getting harder with smaller packages, and the more brutal lead-free soldering temperature profiles.

High-speed parts are affected by the package. Engineers first started seeing this with ground bounce in discrete logic digital ICs. The inductance of the lead frame caused the ground reference to "jump" in voltage when fast pulses were made by the IC. This was yet another reason to love surface mount parts. The smaller physical size meant less inductance in the leads, so unlike early precision analog parts, high-speed digital parts improved in the surface-mount package.

This package card from Texas Instruments is the nicest one I have seen. Sorry to say I gave it away before taking better pictures of it opened up. It had dozens of logic packages and was a big help in understanding the different packages for higher pin-count parts. If anyone has one, take some pictures of the inside, and mail them to me, and I will add the pictures to this blog. TI was a major innovator in package design, and the folks that first identified the ground bounce issue decades ago.

 Texas-Instruments-logic-packages

Figure 4: Texas Instrument package card

As the semiconductor industry matures, I hope that the cult of the IC designer dissipates somewhat and companies realise that all the engineers on the team are rock stars that bring an extremely complex and advanced product to market. This includes the MEs that design the package, the systems-level apps engineers, the test engineers, and the manufacturing engineers. It takes a village, indeed. In case you don't think mechanical engineers have it just as tough as IC designers, here's a story about a friend who worked at a large web server/router company.

My pal Andy Masto, now a consultant, was told by his management that the giant custom CPU in the server would dissipate 228W. This was a critical spec to him, since he had to design all the cooling and airflow. He knew his management was, well let's call them optimistic, since clueless is a rude word. So rather than trust the 228W number, which Masto felt was too low, he walked over to the power supply design group. He asked them how much CPU power they were told to design for. They responded, "318 Watts." That sounded a lot more reasonable for the level of performance Masto knew the new processor would have. His problem was that the BGA (ball-grid array) package was huge. He had to keep the temperatures as low as possible, or the difference in thermal expansion between the BGA and PCB would cause the IC to rip itself off the board. Masto thanked the power supply group for tipping him off, and advised them he would need more power for his fans. As it happens, the CPUs came in at about 310W C much more than the original 228W estimate. This is one of the important skills of systems people, to keep up communication between the groups so that they converge on a design that works.

When I called Masto to refresh my memory about this event, he noted, "The problem with modern BGAs stems from die getting bigger as well as the need for more solder balls to handle the I/O, power, and ground for these massive chips. Die are now 20 to 25mm on a side, and often require 2500 solder balls. As a result, the BGA packages have grown to be 55mm square, and that causes many packaging and thermal problems." These problems with solder balls got me to realise why Intel puts its commercial CPUs in packages with pins. The pins in a socket give the compliance needed to allow for thermal expansion of the part.

Masto also told me about another problem. He said that with these high-power parts, you need very high pressure on the heat sink to get acceptable heat transfer out of the package. The problem is that these high pressures cause the solder balls to flatten over time, a phenomenon Masto called "creep." Eventually, adjacent balls will touch, which is called "solder bridging." Masto notes, "The BGA solder ball pitch is commonly 1mm (0.0394in) and the balls are around 0.65mm (0.0256in). With so little space between them, solder bridging due to creep is a real problem and causes failures. Worse yet, it may happen a year or more after you put the system into service. It is very hard to diagnose; you need an X-ray inspection machine to see it." Masto uses non-linear finite element analysis to optimise the heat sink mounting and avoid solder bridging due to creep.

 BGA-solder-bridging

Figure 5: Solder bridging on BGA.

Just getting the BGA mounted right in production can be a hassle, especially with the higher temperatures needed for lead-free solder. Dr. David Bernard, product manager X-ray systems at Dage Precision Industries notes that moisture in the package can cause "popcorning" as the water turns to steam. This expansion bows the package, and that causes the solder bridging you see above. And bridging is just one extreme. You also have to make sure all the balls contact and solder of the pads on your PCB. This is why planarity is so important on PCBs that have BGAs on them. HAL (hot air leveling) may not be flat enough for large BGA to solder successfully. You may want to specify gold immersion coating, so you can take advantage of the natural flatness of the PCB laminate. Experts like Ken Bahl's team over at Proto Express can steer you right about high-performance PCB design. You should talk to your fab house before you do the PCB design, and also make sure your assembly house has X-ray inspection. A full-service fast-turn assembly shop like Screaming Circuits will make sure they can not only mount the BGA, but test it, and rework it. They are right down the street from Sunstone, another fab house that can handle complex boards.

Like I said, don't go around thinking the other guy's job is easy and only you have it tough. Everything about high tech is hard. That is why you have to make sure everyone on the team knows where the design is headed.

- Paul Rako
??EDN





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