QFP and QFN Packages: Taking Care of Your Complex ICs
The most common IC package types for larger critical components such as microcontrollers are the QFP (Quad Flat Package) and QFN (Quad Flat No-leads) packages. These options are often selected over the more complex Ball Grid Array (BGA) package type because of the relative difficulty involved with escaping traces from a high-density part for BGA Assembly. QFP and QFN packages both have all of their pins exposed around the outer edge of the part footprint, allowing for easier routing without the need for costly special requirements like Blind / Buried Vias or Via in Pad.
The advantage of simplicity in routing is shared by both QFP and QFN parts, but these two options also have some important differences that can impact the overall PCB Assembly Process. First and foremost among these differences is the fact that QFN parts require X-Ray Inspection after PCB Assembly in order to verify the quality of the solder joints. Since the contacts on a QFN part are fully underneath the body of the IC itself, regular testing methods such as Visual Inspection and Automated Optical Inspection (AOI) are not suitable for a proper verification. The need for X-Ray Inspection increases the cost of assembly for QFN parts compared to their QFP counterparts.
Even though QFN packages require some additional work in testing and verification, this can be a good trade-off in many cases for the fact that QFN packages are universally smaller than their QFP equivalent parts. The smaller form factor of QFN parts can allow for a smaller overall board size on your project, which can be necessary in order to meet design requirements, but also has an advantage in PCB Fabrication. The cost of production for your bare PCBs depends to an extent upon the size of the board in question, so if a QFN package will allow you to make your board smaller in the long run, then the decreased cost of fabrication will offset the cost of X-Ray Inspection.
We receive some questions about the difficulty associated with PCB Assembly where QFP and QFN packages are concerned, so we would like to assure our clients that this is a non-issue. In Automated PCB Assembly, these two packages are practically identical, and Reflow Soldering does not particularly favour either package. Some designers are concerned that the small amount of additional solder required for a QFP package will add up in larger volume orders, but this is a miniscule difference that will never affect the price of your quotation.
One last thing to keep in mind when deciding between QFP and QFN package types is the potential for rework. The exposed leads of a QFP package are much easier to deal with than the hidden contacts of a QFN part. With some care, it is entirely possible to solder or desolder a QFP package by hand, where QFN packages absolutely require the use of a reflow oven.
We sincerely hope that this article has cleared up any questions that you might have had about QFP and QFN package types, but we are always happy to talk about any comments or concerns you might have for us on this topic. To get in touch with us about these questions, or to send in your PCB Design Files for a quote, you can Contact Us any time over email at firstname.lastname@example.org, or call us toll-free at 1-888-812-1949.