Designer's Notebook
Rigid-flex brings the best – and worst – of both worlds.
Combining all aspects of a flex circuit with a rigid board that makes full use of HDI techniques is one of the breakthroughs of our time. The stacking connectors for board-to-board or the typical flex circuits are bypassed. If you've ever tried to connect a flex circuit to a stacking connector, you know that's a bottleneck in the process – blindly positioning the flex connector over the mating connector can be fiddly to the point of destroying the connectors. Now what?
Rigid-flex projects remind me of digital/analog projects: the best of both worlds and the worst of both. Just for starters, if the team is taking this route, you know they are serious about holding things together with all possible integration. Both technologies are well understood on their own, though the rigid camp is larger and better understood.
Flex circuits on their own. Flexible printed circuits (FPCs) require more than a change of materials from their stiffer cousins. Additional tolerance must be designed into the data. Reason: The different types of material stacks used in the manufacturing process. For the most part, a flex will also have a rigid section where the connector is mounted. The stiffened area could also be extended to host the ESD protection, an LED or microphone; we're flexible.
Put the pedal to the metal in your PCB layouts.
Printed circuit board design evolves over time and the rate of the evolution is not slowing. High-speed digital design becomes a key topic as we move forward.
By the time you read this, I’ll be officially old. Early retirement age is 62 and I was born in ’62 so I’m eligible for those senior discounts. One of the things that happens as we age is time seems to go by even faster. We’ve seen so much, less is novel. We’re not plowing new furrows but rather deepening ones we’ve tracked before.
I say that to say this: Circuits keep switching faster all the time. What worked in my early days no longer gets it done. My first computer didn’t have a hard drive. It had two floppy disk drives, and they were the ones with 360KB rather than the smaller, stiffer 1.4MB. I had DOS on one floppy and a modem driver on the other. The baud rate was a blazing 1,200B.
Which PCB technologies are best suited to survive 100 years?
The Goal: Build an electronic device that will outlast everyone currently living on Earth. Looking back 100 years, few of us were here and the same will be said in the year 2124. Just reflecting on the brevity of life but we will take a century as forever.
One hundred years ago – 1924 – was the year that the Computer-Tabulating-Recording Company rebranded itself as IBM. Electric blenders, vacuum cleaners, traffic signals and television are among the inventions of the period. Two inventors of the era were leading us toward printed circuit boards though their patents were not commercially successful. Time would prove them to be quite insightful.
Looking back to move forward. PCBs finally took hold around the middle of the century while integrated circuits followed another 25 years later. My "forever" board is going to make use of these early transistor-to-transistor logic (TTL) components that predated complementary metal-oxide-semiconductor (CMOS) technology. The physically larger transistor gates and the 5V logic are a concern. Both types were used on the Voyager space probes to build the guidance and other systems. There was also a fully discrete version of the computer as a backup to the backup. I have confidence in those old Texas Instrument parts.
What’s best for your design may not be what’s best for assembly.
Printed circuit board assemblies animate a collection of components designed to do something useful. Joining those components on a board that completes the connections with a circuit pattern is the best solution we have to create modern electronic devices. The performance and reliability of the device is largely determined by interconnections on the PCB assembly.
The placement itself is a function of the signal connectivity on a local scale and voltage domains on a macro scale. More chips equal more voltage domains. Each IC requires dedicated support consisting of some or all of the following:
- Passive components that do the grunt work of supplying and filtering power
- External clocks for optimizing data flow
- Local power supplies
- Test points, connectors, etc.
- Wherever the I/O pins take you in terms of neighboring components.
Making a board producible is entirely in your hands.
The goal of a good documentation package is that it is complete and coherent enough to proceed with the job without any explanations, waivers, errata or feedback of any kind. It doesn't always work that way, particularly when more than one vendor is involved. The fab drawing is more of a baseline from which they will all deviate to one degree or another. Even using the same vendor all the time is no guarantee that the DfM data come back immaculate.
The foundational aspect of PCB fabrication is a plausible phototool. That final imagery is derived from the artwork that you sent their way. What we know as global micro editing is where the phototool is crafted from the ECAD data. The artwork is more like a starting point.
Etch compensation – pre-distorting the artwork.The first item on the CAM operator's list is dealing with etch compensation. The traces and other geometry that appear on the board are what's left behind after the etch process. First, they must drill and plate the holes with copper. That's done prior to etching and adds a measure of copper to the entire panel. Only then do they mask off the circuit pattern that is not to be removed.
Should the ground connections share vias?
Just like it says "on the tin," wafer-level chip-scale packaging (WLCSP) is a technology that shrinks the substrate down to a size quite close to that of the actual silicon, gallium arsenide or whatever material makes up the die. Rather than calling it a substrate, the WLCSP material is known as a redistribution layer, or RDL for short. It's a subtle but important distinction.
By definition, WLCSP devices exclude wire bonding, leaving flip-chip technology as the method of die attach. That means that there is no die cavity where a solid ground plane on the bottom of the die would normally act as the mating surface. Instead, the chip is mounted face down with BGA-style balls on a pitch that is typically less than 0.5mm. Right there, the challenge can be to maintain a good thermal path through the tiny connections (Figure 1).
The solution is to have numerous ground balls to help dissipate thermal energy. The ground balls can be distributed around the device or gathered into a central square or rectangle, maybe both. Either way, it's best if every one of the ground connections gets a dedicated via rather than combining them to share a via.
Press Releases
- KIC to Exhibit at Four SMTA Expo & Tech Forums in August
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- BTU International Appoints E-Tronix as Its Representative in Six States