The Next Leap of Faith?
Embedded passives are inevitable, and fabricators must be brought up to speed.
There comes a point in every technology’s lifespan when, either by necessity or mandate (i.e., legislation), evolution must occur. We’ve seen this with the migration from through-hole to surface mount and, of course, the recent transition to Pb-free. One such technology that emerged on the scene more than a decade ago, but never quite got its legs, is embedded passives. Not that there wasn’t interest – there was (and is). A relative few leading-edge firms invested in the technology and successfully employed it, but for commercial reasons (and in some instances, patent issues), it was not implemented. Now, however, it seems there is a renewed interest in the technology, largely due to miniaturization requirements. Like SMT many years ago, it appears embedded passives may be the industry’s next leap of faith.
For those unfamiliar with embedded passive processes, I’ll attempt to explain the basic parameters. In theory, it’s a pretty simple concept. At the board fabrication level, a thick film, resistor-based ink, is applied to the various PCB layers using high performance screens and screen-printing equipment. So, instead of having numerous traces that would traditionally have led to resistors on the top or bottom layers of the board, you are now able to locate them on the individual copper layers. The material is printed to a designated area within the layers and then repeated on each layer as required. The key to success is ensuring the tolerances of the prints are within a given range – usually +/-10%. Traditionally, the resultant embedded passive requires a post process (laser trimming) to guarantee the required tolerance. That said, the accuracy of today’s printing systems and the ability for them to verify deposit volumes enable significant reduction – if not outright elimination – of any laser trimming requirements. In fact, our company conducted a series of evaluations on this process and has shown repeatable printing with +/-10% tolerance can be achieved. I’d even take that one step further and say, with a combination of a leading-edge screen printer and well-manufactured screens, a +/-5% tolerance capability should be achievable.
Anything less would likely require laser trimming.
Personally, I think some of the reluctance on the part of fabricators is they are simply unaware of the capabilities of today’s printers. In fact, I know this is the case, based on comments I’ve heard at technical sessions. If board fabs are willing to invest in the tools needed for today’s embedded passives, they can successfully implement this technology.
Rest assured, embedded passives are coming. With the onslaught of 01005s and the next chipset after that, there is going to come a point when miniaturized passives simply cannot be placed or repaired. There is a finite size at which components can be placed at the speeds required, and this barrier will force embedded passives to the fore.
For the assembler, embedded passives are nothing but a win. When you consider nearly 25% of the solder joints on a PCB are for passives and up to 50% of those are resistors (which are likely the passives that will go embedded first), consider all the advantages this technology provides. The board comes pre-loaded. That means placements you don’t have to do; imagine the time saved and throughput achieved by eliminating this from the process. Then there’s the real estate factor. By freeing up the space previously allotted for passives, additional functionality can be added. Statistics also show the reliability of embedded passives to be much better than that of conventional passives. Because they are encapsulated within the layers, these components are less susceptible to shock damage from drops and vibration, as well as more protected against humidity and its ill effects.
Last but not least, embedded technology permits faster clock speeds because they can be placed in a three-dimensional location – the passive device can be placed directly under the chipset; this makes for smaller trace lengths, which enables higher frequency signals to be processed. Consequently, embedded technology is a key enabler to the smaller, faster and lighter paradigm.
For embedded passive technology, it’s not a matter of if, but when. It’s a leap of faith we will all take together and, if your PCB fabricator invests in the right systems, the leap will likely result in a very soft landing.
Clive Ashmore is global applied process engineering manager at DEK International (dek.com); cashmore@dek.com. His column appears bimonthly.
