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The Fundamental Analysis of QFP vs QFN Packages

Do you work with programmable modules, PCBs, or microcomputers? If so, you must definitely work with integrated circuits (ICs) components that function accordingly. When faced with this dilemma, some people turn to the QFP (Quad Flat Package) and others to the QFN (Quad Flat N0-lead) package. But do the two function in a similar manner? Are there significant manufacturability, prototypability, or cost differences between the two? Well, in this article, we shall look at QFN and QFP in detail. Later on we shall then compare and contrast them to see if there lie significant differences between the two. We shall also answer questions such as if the two have any significant differences, which of the two should you consider for your project? So follow along and let us help you make a wise IC choice with all the facts in place, no more gambles!

First off, let us look at the QFN packaging, and then we shall move on to the QFP later on.

QFN package

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 Types

QFN packages come in different variations, which include:

Plastic-molded

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:

  1. A Ceramic or a plastic lid
  2. A copper lead-frame
  3. 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

  1. QFN packages do not have the problem of lead co-planarity
  2. They have tiny footprints; this helps in terms of saving space
  3. These packages utilize regular surface mounting equipment for print circuit board assembly
  4. These packages are relatively thin (they are less than 1mm)
  5. QFNs have incredible thermal performance
  6. Since QFNs are small in size, you can place them close to the board components.
  7. They have impressive electrical performance
  8. Their semiconductor package is not expensive

QFN Issues

Even though QFNs are pretty awesome, they bear some snags, which include:

Manufacturing issues

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.

Soldering issues

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.

Compatibility issues

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.

QFN Assembly

qfn chip

Solder paste printing

The first step of the QFN-PCB assembly process has got to be solder printing. The solder printing process includes uniformly spreading the solder paste onto the PCB. You have to carry put this process before moving on to the placement process.

Placing the component

After solder paste printing, you can then move on to embedding your QFN integrated circuit onto your PCB based upon your PCB layout design. Accuracy and precision are pretty crucial for this part. You can use an accuracy and precision tool to accurately fit your component, even with the excessive interconnection density issue.

Pre-flow inspection

Pre-flow inspection is pretty crucial as you have to ensure that the PCB is suitable or fitting to go inside a reflow oven before you do so. While you are at it, you can take things upper a notch by checking for contaminants on the board’s surface that might hinder the soldering method.

Proceed on to reflow soldering

On confirming that the print circuit board is in worthy shape, you can now place it inside the reflow oven. Remember that you must inspect the PCB thoroughly before this stage.

Board inspection after reflow soldering

In this step, we seek to confirm the quality of the solder.

Also, you require an applicable PCB footprint and a stencil design to assemble this component appropriately. You can now work basing on your intended design with this two in place.

QFN soldering

Soldering QFNs is pretty crucial in the assembly activity. How can we achieve this challenging feat? Well, here is how.

When the print circuit board joins reflow oven, a number of parts start to heat up quicker than others. Why does this occur? Well, this happens due to temperature variation within your reflow oven.

Portions that get heated up faster become lighter, and those that get heated up much later are the ones that have more copper on them. So with that in mind, you can utilize thermocouples to accomplish the whole process and attain better results.

Thermocouples help you monitor QFN surface temperature. They also check the package body temperature does not surpass any typical values.

Rework on an assembled component

If you encounter a QFN defect post assembly, you can rework on that particular component by removing and replacing it. There are unique rework stations available for this specific purpose.

Component found on a rework station include:

  • A split light system – To observe the bottom portion of the QFN package plus the site on the print circuit board.
  • An X – Y table – utilized for alignment
  • A hot air system having top and bottom heaters – utilized to remove components

The reworking process takes place as follows

  • Start by carrying out a pre-bake procedure; this will help avoid any failures related to moisture.
  • Next, you have to de-solder (keep the temperature profile of your component and the board in mind)
  • After de-soldering, you can now move on to mechanically removing the component from your PCB.
  • After removing the component, clean the PCB pads and remove any solder residue left behind.
  • Follow the assembly procedure stated earlier on to mount the next component onto your PCB.

And that is all about QFN. So now let us look at QFP and then, later on, compare the two.

QFP packages

What is a QFP? A QFP is a surface-mounted IC (integrated circuit) package having “gull-wing” like leads extending on either side of their four edges. Generally, these packages are extensively utilized by very large-scale and large-scale ICs. The number of pins utilized in this technology is more than one hundred, making it pretty efficient. QFP technology smoothens IC operations. It also makes these packages pretty reliable when it comes to packaging the central processing unit (CPU).

QFPs have small parasitic parameters, and they are also pretty tiny in size. These features make them suitable for higher frequency applications.

In the manufacturing of the QFP packages, three base materials are utilized, namely:

  • Ceramic
  • Metal
  • Plastic

In terms of quantity, plastic packages take the lead. It is actually so widely used such that whenever you see a QFP package material with no label on it. Then you should automatically know that it is plastic.

QFP basics

QFPs come in two shape variations, rectangular and square:

  1. Rectangular – the number of pins that come out of each side is different due to varying lengths. One side might have more pins compared to another.
  2. Square – they have the same number of pins on each side as they have the same length.

QFP packages have two sections: the top and bottom sections. These two sections are then glued together to form a complete QFP package. QFP pins are usually bent downwards for easier connectivity to the print circuit board. The pins just touch the PCB making the soldering process pretty easy.

QFP integrated circuits vary in terms of formats and also in terms of the number of pins utilized. However, QFPs are often square, and the pin count mostly lies at around 256 or more.

By definition, a QFP with 256 pins means that it would typically have around sixty-four pins protruding on either side of the package. However, some smaller QFPs might have only thirty-two pins, which means they have eight pins on either side. In this example we are assuming that the packages are square in shape.

QFP package variations

As stated above, QFPs come in many different variations, which include:

Bumpered quad flat pack (BQFP)

These QFPs have extensions on each of their four corners. The extensions shield the leads against any mechanical damage before you solder this package onto a PCB.

One major QFP issue is how easily the lead pins get damaged or bent. And because of their fine pitch, it becomes close to impossible to repair a device having bent pins.

Bumpered Quad Flat Pack having Heat Spreaders

These QFPs utilize pin protectors positioned at the device’s corners. And on top of that, it also has heat spreaders which allow higher power levels dissipation to improve efficiency.

Ceramic Quad Flat Packages

These packages utilize ceramic which improves their quality and also their efficiency.

Fine Pitched Quad Flat Packages

These packages are QFPs with a pretty fine pin pitch just as their name dictates.

Heat Sinked Quad Flat Pack

Integrated Circuits might dissipate pretty high levels of heat, especially those which have high pin counts. Due to this heat dissipation this ICs eventually end up having high circuitry levels. The heat dissipated by this IC needs to be ejected from them for better performance. To accomplish this you must replace a couple of pins, often those found in the central part of the opposing side, with pins that are thicker. Solder these replacement pins onto a larger pad on the print circuit board with a large copper area. With this setup in place, this device can dissipate much heat away from the ICs.

Low profile Quad Flat Packages

Low-profile Quad Flat packages or simply LQFPs are based upon MQFP and QFP metrics. They are thinner, having a body thickness of only 1.44mm, which means that they can be utilized in components that have height issues.

LQFPs specs can be defined as follows:

  1. Lead –frame footprint – 2.0mm
  2. Lead count – ranges from thirty-two to two hundred and fifty-six
  3. Body size – ranges from 28 x 28 millimeters to 5 x 5 millimeters
  4. Lead pitches – comes in four different variations: 0.3, 0.4, 0.5 and 0.65mm

Metric Quad Flat Packages

These QFPs have their measurements defined in terms of metric dimensions. Normal QFPs, on the other hand, utilize imperial measurements. They define pin spacing and et cetera in terms of imperial dimensions rather than metric dimensions.

Plastic Quad Flat Packages

These packages are built out of plastic.

Thin Quad Flat packages

TQFPs are a low-profile QPF variation. They have a height of 1mm and a standard lead-frame footprint of 2.0mm. These packages are manufactured using plastic.

Issues to take note of when dealing with QFPs

qfp Packages 

Quad Flat Package damage

QFP pins are pretty small, plus they have small spacings. Their positioning and size makes them vulnerable to damage that is hard to fix. To make sure that these devices are safe, you should store them carefully to minimize the chances of damage. If you plan to transport them, we recommend that you pack them on special ‘waffle’ packaging for adequate protection.

Print Circuit Board density track

The number of pins that a QFP can accommodate means that you should take a lot of care when designing PCBs. If you get clumsy when designing your PCB, then you might encounter track density challenges around the QFP. So careful designing and routing are pretty essential to ensure that you do not violate any designing rule.

Advantages of using QFP

  1. You can utilize sockets
  2. It uses mature technology

Square QFP packages are pretty attractive to a lot of users. They bear one significant advantage that sets them apart from rectangular QFPs:

  • They let the QFP package bear high density compared to rectangular packages

Disadvantages

  1. These devices have a 500MHz I/O limit
  2. Not enough input/output complex chips

Since we stated one significant advantage of using square QFP packages, how about a disadvantage to even things out:

  • During transportation, these QFPs are damaged easily compared to rectangular packages

QFP PCB Assembly

For QFP-PCB assembly, follow the following procedure:

Solder paste stencil

Apply soldering paste onto the PCB metal pad by stencil printing. TO determine the soldering paste volume to use, you should check on the stencil thickness and the stencil aperture. Note that excessive soldering paste causes solder bridging. On the other hand little soldering paste reduces solder spreading. The measurement should hence be pretty accurate.

Solder paste

Solder paste (PB-free) is composed of some type of SnAgCu alloy. Particle size of this alloy must be suitable for solder-stencil apertures dimensions printing.

It would be best to utilize type 4 pastes for this procedure as they are more effective. Also, note that soldering paste is sensitive to humidity, temperature, and age.

Placement

Self-alignment that occurs because of the surface tension of the liquid solder support reliable solder joint formation. However, you still need to place the component carefully. Setting the package manually is not recommended. Instead, we recommend that you utilize a pick and place machine to ensure accuracy each time.

Reflow Soldering

In print circuit board QFP assembly, you can utilize a force convection oven for reflow soldering. Soldering your QFP in an atmosphere full of nitrogen can improve the solder joint’s quality. However, this is not necessary for the creation of reliable joints.

QFP solder joint temperature can be affected by a couple of things:

  1. Its position on the PCB
  2. It’s surrounding
  3. The PCB thickness

For power packages whereby shortage and leakage current below the QFP are a factor to, you should solder using less flux spread.

Remember to follow the manufacturer’s guidelines when soldering to achieve an optimal output.

Now let us advance on to the next section, where we will compare QFPs and QFNs to identify the difference between the two:

Differences between QFP and QFN

Here are some differences that will help you differentiate between these two devices.

Lead difference

QFN – Leads extends on all four sides of the QFN package

QFP – Leads extends out in a gull-wing shape or an L-shape

Assembly portion difference

QFN – The average footing for QFN packages occurs during the PCB assembly procedure

QFP – Lead form has an excellent base for the QFP package even during the PCB assembly procedure

Pin difference

QFN – These packages have only eight pins in total, plus a thermal pad

QFP – These packages, on the other hand, have multiple pins ranging from eight pins per side to seventy pins per side

Conclusion

Quad flat No-lead packages and Quad flat packages are both pretty impressive devices. However, these two are not one of a kind. We hope this article has shed light upon any questions you might have had regarding QFPs and QFNs and their differences. With the knowledge gained from this article, you can make great IC choices without breaking a sweat.