QFN (quad flat no-lead) package is a semiconductor set connecting ASCIC to Printed circuit board (PCB). To achieve this, QFN utilizes SMT (surface-mount technology).
QFN is also a lead frame-based package known as a CSP (Chip Scale Package) because it lets you contact and see lead even after assembly. However, the copper lead framework utilized in the process makes up for the QFN PCB die assembly and interconnection packages. QFNs can also only have multiple or single pin rows, not both.
The single row configuration QFN packages are formed using the following processes:
The saw singulation processor
The punch singulation process
Both of these procedures split up an extensive package collection into solitary packages.
As for the multiple row QFNs, they undergo copper etching processes to produce the number of rows and pins preferred by the manufacturer. After this process, a saw shall cingulate the formed pins and rows, and then you will have a multiple-row QFN.
Furthermore, QFNs come with an open thermal pad fixed below the package. You can therefore do the direct soldering of the packet onto your PCB when you wish to gain optimum transfer of heat from your die.
QFN packages come in different variations, which include:
The plastic molded QFN is, interestingly, one of the cheapest QFN that you can find in the market. It does not have any lid, plus it is only composed of two sections:
The copper lead-frame
The plastic composite compound
However, these QFNs applications lie in the range of 2 to 3 GHz.
The Air-cavity QFNs
Just as the name dictates, air cavity QFNs features an air cavity in their package. These QFNs are composed of three sections, namely:
A Ceramic or a plastic lid
A copper lead-frame
A body molded using plastic (opened and without any seal)
These QFNs are pretty pricey compared to other QFNs due to their construction. However, they are worth the money as they have a broader application scope; they can handle applications that range between 20 – 25 GHz.
Wettable Flanks QFNs
“Wettable Flanks” QFNs have an elevation reflecting solder wetting. Therefore, as a designer, you can visually check to ensure that the pads are appropriately mounted onto your PCB.
The Punch-Type QFNs
Punch-type QFNs have their package molded into a single-mold-cavity set-up. Then, a punch tool splits the molded cavity. So now you know why it is known as a punch-type QFN. However, you can get one package molded up using this method due to this construction procedure.
The Sawn Type QFN
These packages involve the utilization of a mold array process (MAP) for molding purposes. The MAP process involves cutting one massive box set into smaller chunks or parts. After that, sort the sawn types to conclude the process of creating a sawn-type QFN package.
Flip Chip QFNs
Flip-chip is less expensive molded QFN package that utilizes flip-chip interconnectivity onto copper lead frame.
Since they have a shorter electrical path, they are ideal for QFN electrical applications.
Wire Bond QFNs
These packages connect directly onto an IC (integrated circuit), semiconductor, or PCB tracks. They connect to these components using wires connected to the chip’s terminal.
Advantages of using QFN packages
QFN packages do not have the problem of lead co-planarity
They have tiny footprints; this helps in terms of saving space
These packages utilize regular surface mounting equipment for print circuit board assembly
These packages are relatively thin (they are less than 1mm)
QFNs have incredible thermal performance
Since QFNs are small in size, you can place them close to the board components.
They have impressive electrical performance
Their semiconductor package is not expensive
Even though QFNs are pretty awesome, they bear some snags, which include:
If you are a PCB designer, then you probably know that QFN manufacturability is a crucial factor to consider. Even though QFNs are pretty efficient, PCB designers tend to have an issue with them. See, when it comes to reducing fault rates in reflowing and placement, they tend to encounter some challenges.
QFNs perform well when they hit high-volume, low-mix products. However, when they encounter a low-volume, high-mix situation, things tend to become a little messy. What’s even worse is that this problem seems to affect two major areas:
Therefore, when dealing with stencil designs, you must have accurate stencil thickness and aperture design. If this two are not accurate then the results will be catastrophic. For example, if you utilize too much paste or voiding, this will significantly affect the stencil design. Therefore, it would help if you stuck strictly to the guidelines provided by the manufacturer. In this case, the soldering thickness should lie between 2 – 3 mils.
The aperture-pad ratio should also be 0.8:1 or within that range for optimum results. Also, make sure the bond pad design lies at a range of 0.2 – 0.3 away from your package footprint.
Since QFN packages have narrow pad-to-pad pitches, this poses a soldering issue known as solder-bridging. Also, because QFN packages do not have lead, you might face some challenges when you try to desolder these packages.
QFN packages might suffer from dimensional changes on the part or board in which operate on. Why does this happen, you might wonder? Well, this happens because QFN packages have no lead in them. They hence become less robust whenever they experience some nominal CM or OEM practices.
Another dimensional change suffered by this package is board flexure. What this means is that whenever you subject these packages to activities such as board attachment, in-circuit testing, et cetera. Then you are placing them under pretty high stress. Why does this happen? Well, this happens because these packages do not have flexible and long copper leads.