QFN Package: Understanding What It Is

Introduction

Because it is thin and inexpensive, the QFN package is a widely used package choice for modern electronics. It has a simple design and is widely used by PCB designers and engineers known for its simple design and reliability. In this article, we will explain what is QFN Package, how it works, its benefits and where it is used.

What is a QFN Package and What Does It Stand For?

A type of IC package is a QFN package, or Quad Flat No lead package. It is small and lead free designed. Instead of pins, it has got flat contacts around its edges. This keeps things compact, and easy for mounting on a PCB. An exposed pad is underneath the package. Both cooling it and improving its performance. Its small size makes it space savers on tight circuit boards. Furthermore, it’s lightweight and easy to use. Its popularity in high performance devices is a result of these features.

• Punch-Type – This package is molded in a single mold. You separate it using a punch tool.
• Sawn-Type – These packages are produced in large molds. The mold holds bundles of packages together. A sawing tool cuts them into single pieces.
• Wire Bond – Here, wire bonds connect the die with the package. The process best suits standard applications requiring good connections.
• Flip Chip – Connect die directly to the package. Here, it does not rely on wires.
• Thin – Type of packages that are slim and lightweight. Ideal for devices like phones or tablets.
• Micro – These ones are smaller than the thin ones. They are found in wearable and IoT devices.
• Air Cavity – It contains an air gap inside. It has a cooling effect and can never cause thermal stress.
• Wettable Flanks – These QFNs come with side pads for soldering. It provides for inspections and assembly for pads.

What’s Inside a QFN Package and Its Parts?

A QFN has four basic parts. Every component of it has a in how it works.

• Lead Frame – This provides the base that holds everything together. This part links the chip to the PCB for a signal flow. It also carries heat in that it takes it further away from the chip.
• Single or Multiple Dies – The die is the brain of the QFN package. It’s the small silicon chip inside that does all the work. Some packages have one die, while others have several. These dies send and receive electrical signals to the PCB.
• Wire Bonds – These are very fine wire conductors of a QFN package. It connects die to lead frame completing the circuit hence it acts as a short wire and these wires, normally have copper or gold.
• Molding Compound – The outer covering of the package. Protects against damage and corrosion to the delicate parts. Also provides insulation to prevent electrical problems.

QFN Package Working Principle

The PCB is connected to the IC through QFN package. The copper lead frame has IC sitting on it. These electrical signals are carried from the IC by the frame. The terminal pads on the bottom are where these signals travel. The PCB also has the pads soldered directly on to it. This is strong and reliable. The performance is improved by the no-leads design. The package has no protruding leads and hence reduces signal delay.

• QFN Package Heat Dissipation

Thermal dissipation in QFN packages is very important. They’re made from plastic, so the body helps to move heat away from the IC. It keeps the IC cool and keeps it from being damaged. The heat is spread by the copper lead frame. It transfers heat from IC to the package. The PCB is connected to the bottom pads on the terminal. The PCB is having thermal vias that can help to move heat faster. Other cooling tools, such as heat sinks, may be used.

• QFN Package Electrical Performance

Due to its excellent electrical performance, particularly in high speed applications, the QFN package is highly valuable. Low inductance and “no leads” design results in low signal delay and high signal integrity. Elimination of protruding leads makes the QFN have shorter signal paths with resulting increase in efficiency. Furthermore, performing electrical signal conduction of IC to the terminal pads results in copper lead frames better performance. These pads are direct at the bottom of the package and connect to the PCB with the guarantee of a solid connection.

Different Terminal Pads of QFN Package

Fully Exposed Terminal Ends

These make the terminal pads all the way to the edge. This design allows for better access to be able to solder for a strong connection. Visual inspection of the solder joints is also made easier. There is no hassle in spotting any issues during the quality checks due to which you have clear visibility. Ideal for reliable soldering and easier maintenance. That is because of their exposed ends.

Pull-Back Terminal Ends

These have their pads set a little away from the edge. This leads to a tiny space between your terminal and the package molding. The design eliminates the formation of a visible solder fillet at the edge of the lands. It also helps in reducing the risk of solder bridging with another terminals. It keeps solder joints much cleaner and more reliable during the assembly process.

Side Wettable Flank Terminal Ends

These were designed for better solder stability on the sides. Specialized treatment is applied to the side terminal edges to promote solder wetting. This makes for easily inspected visible solder fillets. It’s much easier to check the solder joints with automated optical inspection (AOI) because it can do that job quickly.

QFN Packages Advantages and Disadvantages

QFN Advantage

Lightweight and Easy to Handle

These are light weight packages easily handled. They are small and easy to put on a PCB. You’ll need no additional handling tools for assembly. That makes them usable in manual and automated process.

Thin Profile and Small Form Factor

It is slimmer and compact. It saves space on the PCB. This permits one to place more components in a smaller area. In other words, it is best for devices when size is the limiting factor.

Short Bond Wires for Die Connections

QFN packages have short bond wire die. The electrical performance is also improved by the shorter connections. Major benefits are reductions in delay and signal integrity.

Low Lead Inductance

They interfere less and have more signal flow due to QFN packages. This ensures keeping electrical quality of signal. Lower inductance is key to fast and reliable performance.

Excellent Heat Dissipation

The IC is QFN package which exposes the pad to help pull heat out of the IC. It keeps your components cool, and functioning fine. It’s the perfect application to keep heat off from anything in use.

Affordable and Widely Available

QFN packages are very easy to find and are cost effective. And it’s cheaper than many other package types. And most importantly, they give great performance and they are very affordable.

QFN Disadvantage

Difficult Alignment

The assembly alignment of the QFN package can be difficult. The small terminal pads should match the PCB. One requires proper connections and preciseness to place them. If you have a small misalignment, you’re going to have problems.

Precise Solder Paste Application

A QFN package requires the amount of solder paste to be correct. Too little paste may also be the cause of weak connections. If there’s too much paste, it can also cause short circuits. Careful control during the soldering process is required to do that.

Limited Visibility of Bottom Terminal Pads

The visibility of QFN package bottom pads is a challenge. It’s not possible to compare soldering quality. You have no possible way to see into the joints visually without some kind of special tool. It can be complicated to inspect.

Reliance on X-Ray or Electrical Testing

You can’t see the bottom pads very easily and you need extra methods. X-ray testing is obvious to see solder joints. The electrical testing is done to verify of the IC. This is so we don’t spend extra steps for quality assurance.

QFN Package Assembly

Lead Frame Bar Coding

A unique barcode marks each lead frame. Once you’ve put the pack together, this barcode follows it through the entire assembly. This enables to find what component is doing at every step. This barcode makes work more organized and has good quality control.

Taping

This consists of putting QFN packages onto carrier tape. Each package is kept securely in place by the tape. It structures the components that are to be used in the successive stages. Important to prevent damage during movement.

Die Attach

The die is bonded to the frame via a special adhesive. As a result, they form a solid electrical and thermal connection. For best performance it’s important that the die stay securely in place.

Wire Bonding

It connects semiconductor die to lead frame. Very fine copper wires are used to do so. Conductivity and cost effectiveness lead us to use copper. Strong electrical connections for the package are created by these wires.

Molding

This consists of protective plastic surrounding the die and wire bonds. Such applies protection and provides insulation. This contributes to the fact that the internal components are secure because they are molded in an advanced process. It also protects from corrosion and physical damage.

Detaping

Taking the tape from the package very carefully. The tape is removed by specialized equipment that does not damage the package. It must be done precisely or component parts can be damaged.

Singulation

Used to separate individual QFN packages from the carrier tape. With a shear or saw it can be done. This means that the QFN packages are all ready for mounting onto a PCB.

Soldering a QFN Package to a PCB

• Step 1: Remove the Original Chip

Initially you will preheat the PCB while applying hot air. It also helps to loosen the original chip and subsequently remove it. When the chip comes off, you have to clean the pads. The cleaning of the pads makes sure the new chip is properly attached.

• Step 2: Clean the Pads

You then need to clean the PCB pads after removing the original chip. Use some flux and a solder wick to clean up any left over residue. The purpose of this step is to make the surface ready for new QFN package. Clean pads prevent future soldering problems.

• Step 3: Apply Solder to the Radiator (if required)

Apply a small amount of solder if your design is going to need a radiator. It makes a solid connecting for the heat to be dissipated. Do not use too much solder or it will short. This step just needs a tiny, controlled solder dot.

• Step 4: Position and Fix the Device

Then place the QFN package onto the cleaned pads. Make sure the pins go slowly into the pads on the PCB. Gently press it down using tweezers as soon as it’s in place. It secures the device ahead of soldering.

• Step 5: Solder the Perimeter Pads

So, now start soldering its perimeter pads. Touch each pad with a lightly tinned soldering iron. If you do this correctly you will see small solder beads form on each pad. Please make sure the PCB doesn’t get out of alignment when soldering.

• Step 6: Final Soldering and Inspection

Then check your work very carefully. Make certain that all solders joints have been soldered appropriately. You will need to check and make sure that any joints that did not look good are repaired or reworked.

QFN Package Applications

These types of packages are widespread in the numerous consumer electronics, owing to their effectiveness and compactness. Here is a list of product that is using QFN packages.

• Digital Cameras
• Laptops
• Mobile Phones
• MP3 Players

More Frequently Asked Questions About QFN Package

• Why Is the QFN Package Widely Used?

It’s small and efficient, therefore popular among all. It is a perfect right-sized memory for applications that require to fit in small PCB with limited space. The electrical performance is reasonably good whereby signal delay time is reduced and the signal quality is improved. Also, the thermal behavior of the material is good to release heat. It is also economical and therefore most suitable in all the above applications.

• How Are the QFN Solder Joints Inspected?

Underneath the package are the QFN solder joints. As a result, they are difficult to examine directly. Typically optical inspection has been used to check these joints. This method can’t always detect everything, however. That’s why X ray inspection is also used. It works like a window through the package. In this manner, solder joint problems, if any, can be found.

• Do All QFN Packages Have a Ground (GND) Connection on The Large Bottom Pad?

Not all QFN packages have GND connection on the large square (bottom) pad. This pad is usually used for helping dissipate heat. It will usually attach to a thermal plane for better cooling. In some cases, however, it may be also tied to ground. This is mostly found in high speed applications such as RF chips. Check the datasheet for the specific chip and always make sure how to deal with this pad. Not all QFN packages use it the same.

• Are There Any QFN Dummy Packages for Practice?

There are no dummy QFN packages for practice. But cheaper chips exist in QFN packages. Additionally, ATmega324P and ATmega164P are low cost ones. They are useful, and are also packaged in the 44 pin QFN like that of the ATmega1284P. It also does not exclude QFN chips in small packages, you will find even voltage regulators in your everyday devices. But all that means there isn’t room for a 44 pin QFN, and there are still loads of cheap QFN parts to practice on. If you buy in lots or even buy single pieces of it, you can get these for a reasonable price.

• Are There Any Pre-Tin QFN Package?

Most QFN packages already come pre tinned. So, these are the solder pads, they have a matte tin finish. This finish makes it easy to solder pads. It will work with either lead free or traditional solder. Lead free compliant is normal for pre tinned QFNs. With this feature, there’s no need for extra steps, which saves time. They can be soldered directly to the PCB.

Conclusion

To summarize, the QFN package represents a compact and efficient solution for current day PCB designs, for its low thermal performance and excellent electrical performance. We at PCBMay are PCB manufacturers enabling you to create and quickly receive high quality PCBs of QFN packages or any advanced PCB designs. Leave your projects in our care for reliable, cost effective and precise PCB solutions.