Package on Package (PoP)

Wednesday, November 24, 2010

Electronics Manufacturing Trends- Package on Package

Welcome to Lean Streams's Blog. Our purpose is th share World Class Manufacturing Trends- Technical and Financial.
We have decided to use "Package-on-Package placement(PoP) as our Initial Electronics Manufacturing Trend to Share and will explain how to achieve PoP placement with Fuji's NXT and AIMEX Platforms. Many of our customers are already manufacturing their products in N. California using PoP techniques with the NXT platforms. We hope this information in the article will be of some use to you.

Package on Package Trends in Placement Techniques

2.Trends in Placement Techniques
The trend for high density placement techniques is moving from QFP and BGA placement to Package-on-Package and Water Level Chip Scale Package (WLCSP) placement.PoP is especially becoming an essential technique to efficiently place Application Specific Integrated Circuits(ASIC) and memory onto panels for cell phones and digital cameras. The main reason that PoP techniques are used is that ASIC or memory which has passed testing can be handled independently. The use of ASIC or memory for general purpose products reduces stock parts and creates greater freedom for design. This is a hard to achieve with System in Package(SiP) but a characteristic advantages of using PoP techniques.

Types of PoP Assembly Techniques

3.What are PoP techniques?
3-D packages are mainly split into two types- one uses embedded techniques and the other uses layered techniques.
The embedded technique is a construction method for embedding passive parts such as resistors and conductors, and active parts such as transistors into the panel during the manufacturing process. The layered techniques is a construction method which uses layers of bare chips or layers of packages. The term Package-on-Package is used for the package layer construction method. Furthermore, there are two construction methods within PoP called "Pre-stack" and "On-board stack". To achieve Pop placement requires high level of placement technique and high performance process equipment.

3.1 Pre-Stack
The bottom package is set on a carrier jig and flux(solder paste) is applied to the top package which is then placed on top of the bottom package. They are then passed through a reflow oven to join the two layered packages together. The "Pre-stack" package is then used in the panel placement process as a single part. The part is placed on the panel and passed through a reflow oven again to bond it to the panel.

3-2.On-board Stack

Solder is printed onto the board and then the bottom package is placed onto the panel (as is the same as SMD surface mounting). Next flux(solder) is applied to the top package which is then placed on top of the bottom package. The panel is passed through a reflow oven to join the panel and bottom package, the bottom package and the top package together at the same time. On-board Stack requires only one pass through a reflow oven.

3-3.Advantages of Pre-stack and On-board Stack

Previously, it was possible to efficiently reduce the placement area by creating layers of placed packages separete on the panel surface. Both Pre-stack and On-board Stack have their own characteristic advantages which depend on the construction method. Flexible panel design is made possible due to the freedom in combining the shape and form of the package.

Pre-stack advantages
.Reliability is guaranteed for each layered package
.Can be performed at standard SMT lines
.Suitable for large scale production

On-board Stack advantages
.Can support flexible part combinations
.Specification changes are possible in a short time span
.can help to minimize work in progress inventory

Important Points for PoP Techniques

4. Important Points for PoP Techniques
The diagram below shows a collection of characteristic factors that affect the quality of PoP placement.

4-1.Defective Joining Due to Package Warp
The most important factor in the above diagram is package warp. The compatibility of the top package and bottom package is very important because the type of warp depends on the package. The package height must be controlled,especially because the bottom package is extremely thin. Since there are various types of package warp, it is important to know the warp characteristics of the top package,bottom package as well as the panel.
The above drawing shows examples of actual checks for warp behavior and compatibility between the top and bottom package. For the 12mm example,the top package warps upwards but the bottom package is warped upwards before reflow and changes to warp downwards while passing through reflow. for the 14mm example, the top package warps downwards but the bottom package is warped upwards before reflow.
Further to this the bottom package warps upwards at room temperature which is large change from the time it was warped downwards at the time of reflow.
Recently, it has become possible to control these warp trends to a certain degree at the package creation process. However, it is unavoidable that package warp changes occur due to heat, and people who use packages must recognize that the levelness of packages is different for room temperature and during reflow. The changes during reflow revert back to their original state at room temperature. However, it is important to be aware that the larger the part size, the easier the past size, the easier it is for the warp to be more significant during reflow.
The above pictures show examples of defecting joining. Look at the bottom package. It is easy to see that the shape of the joining area for the package center and edges is different.
Looking at an enlarged picture, the solder on the panel and bumps on the bottom of the package are not touching. A possible explanation is that the bottom packaged warped and was raised above the pads on the panel during the reflow process.

4-2.Transfer Amount
An adequate amount of solder must be transferred to join packages. If too much is used,shorting will occur. If too little is used, open defects will occur. The transfer amount depends on the dip depth during the dipping the dipping process. The dip depth is achieved using a mechanism that pushes the bumps down onto a transfer platform on a dip unit. This means that the transfer amount depends on the film thickness of the flux or solder paste. From experience, no problems will result of the thickness is set to be approximately half of the bump height. For example, the PoP top package has a bump pitch of 0.65 mm and bump diameter of 0.45 mm with a bump height of 0.40 mm. If the dip depth is set to 0.20mm, an adequate transfer amount can be achieved. As shown in the diagram below, a small cmount of flux has crept up the bump. However, after considering such things as the bump coplanarity, a setting of approximately 50 to 60% of the bump height should be used.
Other factors that influence the transfer amount besided the dip depth are the lowering speed, the raising speed and the length of time the part is held in the flux. All of these parameters can be specified. However, the transfer amount mostly depends on the dip depth and in testing, the other factors did not have such a large effct on the transfer amount. The raising speed had a small effect on the transfer rate but it was minor.

4-3. Flux Dipping
The pictures below show a side view of bumps with a height of 0.38 mm and flux applied. Each picture shows flux  applied at a height of 0.10 mm, 0.20 mm and 0.30 mm. The line where flux is present is a little difficult to see. However, it was confirmed that flux crept up the bump further than the amount it was dipped in the flux (the flux film thickness). When the film thickness is 0.30 mm, there are cases in which the flux actually reaches the bottom of the package.

4-4.Solder Transfer
Other than transferring flux to bumps on the package, solder paste developed especially for PoP is used. This solder paste is a mix of solder with a particle diameter smaller than normal
(approx.10 u m) and flux. Normal solder paste does not have enough solder particles to sufficiently join the packages. However, the PoP solder paste ensures that a uniform thickness of solder coats the bumps, which is effective in preventing defective joining due to package warp. Although, if too much solder is applied, there is a greater chance that bridging will occur. Therefore, care should be taken not to supply an excessive amount of solder.

The pictures above show a side view of bumps with solder paste applied. The bump height is the same as before, 0.38 mm, and flux is applied at a height of 0.10 mm, 0.20 mm, 0.25 mm and 0.30 mm. Just as with flux, the solder paste also creeps up the bumps greater than the film thickness. In testing, a transfer amount of 0.20 mm was the best for bump heights of 0.38 mm.An approximate amount of 0.07 mm hanging solder paste was present on the bottom of the bump for this transfer amount. However, this hanging solder paste will help to join the top and bottom package together when a gap is created between the top package bumps and bottom package pads due to warping.

4-5.Self-alignment Effect in PoP
Generally, solder paste works more efficiently than flux for self-alignment.

4-6.Process Equipment Requirements
After considering each factor discussed so far, to ensure success in PoP placement, the following conditions for process equipment are required.

[Placement machine]
.High precision head - placement accuracy,Z-axis control
.Productivity - batch dipping with multiple nozzles
.Transfer check function - vision recognition

[Dip process]
.Film thickness adjustment
.Automatic flux supply
.simple operation, maintenance

PoP Placement at the NXT and AIM

5-1.Placement Machines 
5-1-1.High Precision Head - Placement Accuracy Z-axis Control
The G04 head which has a four fixed nozzle configuration can place parts with the same high precision as the H01 head due to its exact mechanical structure. Low load placement(1.5 to 2N) is achieved using nozzle springs which minimize the weight.
Furthermore, the Z-axis speed can be specified and placement height can be controlled to support dipping or stack placement. The NXT and AIM have the advantage that placement heads can be exchanged quickly when introducing PoP placement to a line.
5-1-2. Productivity - Batch Dipping with Multiple Nozzles
The G04 head considers productivity during mass production and supports individual dipping for each nozzle as well as simultaneous dipping for four nozzles. Furthermore, the vision processing and dipping order can be selected to match the customer's needs.

5-1-3.Transfer Check Function-Vision Recognition
The images below are images acquied during vision processing after dipping. It is easy to confirm that the bump on the top right is not coated with solder paste but the remaining bumps are coated with solder paste. NXT and AIM machines can detect whether solder paste has been transfered to bumps. During the detection process, it is easy to specify data acquired for the check based on the actual image.

5-2.Dip Process Equipment
Simply by purchasing and adding a dip flux unit which can be loaded onto a NXT or AIM, dip processing is possible. the dip flux unit maintains the required film thickness by rotating the flux reservoir and using a squeegee. This unit can be used with numerous types of flux and solder paste for transfer to flip chips and shield casing.

5-2-1.Adjusting and Checking the Film Thickness
A micrometer is used to adjust the film thickness at the unit and to adjust the lowering and raising action of the squeegee. When checking the film thickness, use a comb gauge. The dip flux unit must be set on the machine to perform adjustments and checks.
An option is available with the rotary type dip flux unit to automatically change the film thickness.
5-2-2.Flux and Solder Paste Automatic supply
When the amount of flux is the reservoir becomes insufficient, the dip flux unit automatically supplies flux. When the flux reaches the required amount, the flux supply stops and production can resume. Solder paste can also be controlled in the same way.
5-2-3.Simple Operation and Maintenance
Without removing the dip flux unit from the machine, maintenance can be performed for the squeegee and flux reservoir by simply pulling out the unit to a maintenance position. Furthermore, no tools are required to remove or attach the flux reservoir.
5-3.Machine Configuration Settings
PoP Placement can be performed at the NXT or AIM after setting the following three items Fuji Flexa.
  • Setting for flux application
        [Shape Process] - [Flux] - [Generic] - [Do Flux]
  • Mounting slot setting for dip flux unit
        [Configuration] - [Feeder Setup] - [Feeder Name]
  • Specifies package to stack(stack target)
        [Coordinate] - [Stack Target]

    Other selections and changes are available for the conditions of each operation.

        [Accessory Software]
  • Time setting for part vision processing and dipping
  • Parameter setting for stirring flux

        [Fuji Flexa]
  • Parameter setting for dipping
  • Parameter setting for stack placement
  • Setting for transfer check function
       *Only solder paste can be used with vision type 142


To achieve PoP placement, high precision process equipment must be used. The NXT and AIM do not require any machine modification or the introduction of any customized equipment. PoP techniques can be used to mass produce customer products simply by acquiring a dip flux unit.