MCU Packaging: Sometimes It’s What’s on the Outside That Counts

My young children enjoy playing with gift boxes and wrapping paper just as much as they enjoy playing with the toys inside. I can see why. Packaging is interesting, especially in my world of Kinetis microcontrollers. But why? I sat down with our MCU product engineering team to get some of my questions answered — read on and you’ll see why it’s so interesting.

QFPs, BGAs and CSPs … oh my!

A quick review of an online distributor’s website revealed that more than 400 package options exist across various MCUs. That’s a whole lot of package options. But why so many? Engineers have design requirements that can vary just as much as the applications they are designing. Some are designing for smallest size, others are looking for packages that will simplify their manufacturing and reduce PCB costs and some are most concerned about their products long term reliability. Engineers are primarily looking at three criteria in their package selection: type, dimensions and pin pitch. As is the case with many aspects of technology, looking at package types can result in acronym overload. To address the broadest base of customers, Kinetis MCUs are available today in several different package types like: QFNs, SOPs, SOICs, BGAs, CSPs and QFPs. Here’s a rundown of what they all mean.

QFPs

A QFP or quad flat package is a surface mount integrated circuit package with “gull wing” leads extending from each of the four sides. An LQFP is a low-profile quad flat package, which offers a thinner body thickness at just 1.4 mm, which is almost half the height of some traditional QFPs. Kinetis MCUs using QFP technology leverage this thinner body package.

BGAs

In comparison to a QFP, a ball grid array (BGA) package is one where solder balls are placed in an array under the package rather than external leads placed around the edges. This allows more electrical connections for a given area but increases some of the PCB complexity. There are several types of ball grid array (BGA) packages. The wire-bonded MAPBGA (molded array process ball grid array) is an excellent package for low-performance to mid-performance devices that require packaging with low inductance, ease of surface mounting, low cost, small footprint and excellent package reliability.

Extremely thin fine ball grid array (XFBGA) is similar to the idea of an LQFP in that it addresses the concern of thinness or body height, and an XFBGA can have a profile height less than 0.5 mm. Kinetis MCUs are offered in both traditional BGAs and these thin MABPGAs.

CSPs

CSP is short for chip-scale package. According to IPC‘s standard J-STD-012, the package must have an area not more than 20% larger than the die and it must be a single-die, direct surface mountable package to qualify as “chip scale.” With such a broad definition, there are even BGA packages that fall into this category. In fact, I read that there are well over 50 different types of CSPs. Wafer level chip-scale packages or WLCSP technology is a true CSP technology because the package is actually the same size as the die with just an array pattern of bumps or solder balls attached at an I/O pitch that is compatible with traditional circuit board assembly processes. These type of packages can get really small — even smaller than the dimple of a golf ball, as we have showed with the Kinetis KL03. Kinetis mini MCUs use WLCSP technology and are primarily targeted for those applications that really value the miniaturization.

One size does not fit all

Like most people, I purchase a gift item first, followed by the box, bag, wrapping, ribbons and card. However, that’s not necessarily the case with MCUs. Often the packaging is the key decision. It is not uncommon for me to receive a call from sales with something like, “My customer needs a 5V MCU part in a 6 mm x 6 mm BGA — what do we have?” Never a mention of speed or peripheral set, just a generic request on the I/O voltage and a very specific package request. It is clear that packaging is often a leading factor in selecting an MCU. And since MCUs are used everywhere, you can imagine the countless different packaging requirements needed. An engineer building 100 units of a large industrial machine has a different need than someone building 10 million units of a portable device. There are a ton of different opinions out there as to which package type is better, where engineers talk about everything from signal routing and PCB layers to solderability and reliability. However, once they determine the right package type for their project, the criteria of dimensions and pin pitch must also be addressed. The dimensions of the chip have a direct impact on the overall size of the end device. An MCU is 3D of course, but traditionally, designers have really only looked at X and Y dimensions. That is starting to change as people realize that the height of the device is super important too, which is why those thinner packages I mentioned are becoming increasingly important as well. Pitch relates to the spacing between the pins (distance between the centers of the adjacent pins) and it has a direct impact on the PCB layout and manufacturability. For a given sized package, the smaller the pitch, the greater number of pins available. However, the smaller the pitch, the more attention that needs to be given to ensure robust and reliable board assembly. You may wonder why I did not mention pin count as an obvious selection. Well, the truth is that as long as you have the right dimensions and pitch and are able to bring out the right features, the pin count should not matter as much.

Package Your Way Program for Kinetis MCUs

The product and packaging engineering teams have strutted their technical capabilities many times since I joined the Kinetis MCU team, including with the world’s smallest Arm Powered® MCU in 2013, then again in 2014 when they shrank the world’s smallest Arm Powered MCU by another 15% and most recently when they announced the new Package Your Way program , which seems to me a formal introduction of what they have already been doing for years — qualifying and delivering alternative packages for Kinetis MCUs to customers in a relatively short time. When a new product is launched, it is usually launched in a couple of different packages. With the Package Your Way program, additional package options are also now broadly offered. These alternative packages are available for select Kinetis MCU families, with pin out and pricing information readily available for customers. When requested, samples are built and production committed based on customer demand, with these new parts often being delivered to customers in as little as one month. Through the Package Your Way program, you can choose among so many package options to meet your needs, who (like my toddler at Christmas) think that the packaging is every bit as interesting as the actual part on the inside.