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Application Notes: BGA Choosing an Area Array Category

Introduction: There are four categories of area array device vision processing on a Universal Instruments Corp. pick and place machine. These are BGA, BNGA, Variable Pitch BGA, and C4. Each category is tuned to specific geometric characteristics of certain area array devices. When do I use each category? Listed below is a table describing the typical application for each area array-processing category. Category Description of Target Devices Example(s) BGA Any area array device whose

spheres are on regular row and column pitch. This is the most common type of area array device.

BNGA Any area array device has no set pitch. This may include devices that have regions of spheres on common pitch, but not on pitch with other regions. The small groups of bumps on the left not on pitch with the main bump arrays

Variable Pitch BGA

An area array device designed to have a common pitch, but due to its manufacturing process, the sphere locations vary relative to its CAD model. This is typical of a NeXLev™ connector device.

C4 Die area array devices whose

bumps are small, and are typically on no set pitch.

51030001, Rev. C

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Application Notes – BGA

Why use the BGA category? The BGA category is set up to process area array devices with interconnect spheres that are on common row and column pitches. It is optimized for ease of programming and processing speed. To see if your target device fits into this category, see the document entitled “Application Notes - Choosing an Area Array Category”. Programming a BGA device: Generally, sphere locations and descriptions as well as camera and vision parameters must be programmed. See the document “Area Array Programming Rules.doc” for details. The BGA Algorithm The BGA algorithm takes advantage of the regular row/column structure of the spheres on a BGA device. The algorithm has four steps:

1. All sphere features are located, yielding an X-Y coordinate for each sphere. 2. The location of all the rows and columns are located 3. Based on the rotation and location of the rows and columns, the location of the

device is calculated. 4. If selected, all ball inspection is executed. 5. If selected, the row and column pitch are inspected

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FAQs and Helpful Hints: Symptom: My small (low I/O count) BGA is rejected from time to time –

1. Reason: You have a low I/O BGA (not many spheres): The BGA processing algorithm takes advantage of rows and columns that have a large number of spheres. In order to classify a row as a row or column as a column, at least 25% of the spheres in that row or column must be found. If there is a row or column with eight or fewer bumps programmed, only one or two bumps need be found to form a row or column candidate. If there is noise, that could also be mistaken for a row or a column. Other symptoms for this are messages like “Wrong Regression Angle” or “Wrong Projection Angle” displayed in red letters in a black bar at the bottom of the vision monitor.

2. Solutions:

a. If the device fits within a single field of view, re-program the device using

the BNGA algorithm and category. BNGA uses an algorithm that is more robust in the case of low I/O count devices.

b. If your machine is equipped with the C4 category option, re-program the device in the C4 category. Similar to BNGA, the C4 category uses an algorithm that is more robust for the case of low I/O, and also supports multiple fields of view.

c. If the device is larger than a single field of view, re-program it in the pattern category. It is a little more cumbersome to do this, but it will locate the cues.

An example of locating the columns of a BGA

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Symptom: My BGA is rejected constantly

1. Reason: There is probably a large discrepancy between your programmed definition and the actual part that being picked up.

2. Solutions:

a. Wrong part selected from the component database: Open the product and

look at the definition in the component database to ensure that it is the correct part.

b. Sphere size is programmed too large: In general, you should program the size of a sphere to be 75% of the manufacturer’s specified sphere size. Alternatively, use Enhanced Component Setup (ECS) to view the BGA with a camera. Select the draw option to see if the programmed sphere diameter matches the image. If it does not, adjust the programmed diameter until it does. Note: If you are running a CCGA device, the programmed diameter should be equal to the manufacturer’s specification.

c. Lighting is not programmed correctly: Check the component database and make sure that the component is programmed with side light only for devices that have sphere interconnects, or on-axis light only for ceramic column grid arrays (CCGAs).

Symptom: My multiple fields of view BGA is sometimes placed one row or column shifted from where it should be –

1. Reason: As a speed optimization, the BGA algorithm provides a location of a device even if the entire device is not viewable in a single field of view. Because

The same BGA imaged with side light (left), front light (center) and On Axis light (right) Notice how bright spots of the bump images get smaller moving from left to right

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of this, the algorithm may not be able to determine where the starting row or column is. If the pick location is shifted, or the search range (programmed as the pick tolerance in the feeder database) is too small, a row or column of spheres may be excluded from the final correction.

2. Solutions:

a. It is recommended that the field of view of the camera be at least large

enough to include the center of the BGA. This will ensure that enough information will be included to center a device properly.

b. Increase the search range programmed in the feeder database. c. If possible, use a camera that can view the part in a single field of view. d. Ensure that the pick location is as close as possible to ideal. e. Use “All ball inspection”. All ball inspection will ensure that all

programmed spheres are found, and the part will be rejected instead of being placed badly on a board.

f. If the device fits within a single field of view, re-program the device using the BNGA category. This category will ensure that all programmed spheres are found.

Symptom: From time to time, my BGA is placed with a large angle relative to its placement site –

1. Reasons:

a. If a BGA is presented to a camera with a large rotation (> 2 or 3 degrees) relative to ideal, it is possible that bumps that are not in a rows or columns can look to the algorithm like they are aligned in rows and columns. If this happens, an incorrect location will be calculated and the device will be placed with a large skew.

Minimum FOV size

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b. The device may be hitting a component or empty nozzle on an adjacent spindle.

2. Solutions:

a. Increase the “theta pick tolerance” in the feeder database to 10 degrees.

Unfortunately, this may also increase the BGA processing time. b. Make sure that the correct nozzle is being used so that the device is

handled cleanly. c. Make sure that devices or nozzles on adjacent spindles do not hit the BGA

when the head makes a large rotational move. It may be necessary to have the machine remove adjacent nozzles to run this device.

d. Re-program the device using the BNGA category. This category uses a different processing algorithm that is not as sensitive to large rotations at presentation time.

Symptom: When running a NeXLev™ connector, it is rejected a lot, and the placement accuracy is often poor –

1. Reason: The locations of spheres on a NeXLev connector vary from device to device. BGA processing assumes that the relationship of the spheres to each other is fixed, and consistent from device to device. If it is not, there may be accuracy issues, and certain spheres may be noted as missing because they are not in the correct location.

2. Solution: Re-program the device in the “Variable Pitch BGA” category. The

processing algorithm for this category uses a more robust location calculation algorithm, as well as a more sophisticated sphere location and assignment algorithm.

Symptom: Cannot find P2P camera for component

1. Reason: The bump diameter and/or spacing of the component does not meet the minimum pixel requirements for processing using the accessible cameras of the machine configuration..

2. Solutions:

a. A higher magnification of camera may be required. b. Change the minimum pixel requirements parameters in the software via

Machine Configuration>Parameters>Set up Variables>Vision. Note: These values are based on the specified capability of the vision algorithms. By reducing these values, the robustness of the find may be affected.

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Application Notes – Area Array Programming Rules

BGA C4 Variable Pitch BNGA (Flipchip) BGA Introduction: There are four categories of area array device vision processing on a Universal Instruments Corp. pick and place machine. These are BGA, BNGA, Variable Pitch BGA, and C4. These devices share many common programming rules. This document describes these rules and provides some simple examples. General Programming Philosophy: There are four categories of information that you will need to supply to program an area array device.

• Geometric Information: Sphere or Bump locations, and sizes • Imaging Information: What camera do you wish to use and lighting • Algorithm information: Processing categories, and parameters • Optional inspections: All ball inspection, etc.

Programming Geometric parameters for an Area Array Device:

The Model: Area array devices are programmed in the “live bug” position. That is, they are programmed such that the geometric parameters describe the location of the interconnect media (spheres or bumps) when the device is in its normal vertical alignment for placement. (Bumps down!). You need to imagine that you are looking through the device to visualize the locations of each sphere. Programming the Sphere or Bump Geometry: Area array devices generally use spheres or bumps as the interconnect features, and as such are the natural choice for measurement features for placement. The single parameter used for bump geometry is the diameter. This may seem like a simple parameter to

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program, but there is a little more to it than just entering the manufacturer’s provided diameter. You must take into account that the lighting will have an effect on the perceived size of a sphere. When a sphere is illuminated, it is generally done using sidelight. Sidelight provides an excellent means of lighting the spheres, while minimizing the effect of traces and features on the substrate or die. Unfortunately, due to the geometry of a typical sphere, and its interaction with the side lighting, the imaged sphere diameter appears smaller than the actual sphere. The general rule of thumb is to program a sphere or bump diameter at 75% of what is described in a manufacturer’s document. Exception: If the device to be programmed is a CCGA (ceramic column grid array), then the feature that is being imaged is the disk shaped end of a solder column, illuminated with on-axis light. In this case, you should program the diameter to be what is provided in the manufacture’s specification. Although the database provides discrete fields to program the diameter for each array, the diameters are required to be equal. If different values are programmed for each array, the diameter of the first array will be used as the model’s value. Bump Diameter and Spacing Requirements: The specified lower limit for ball diameter is 4 pixels. The specification for minimum space between balls is also 4 pixels. For components that do not meet these guidelines, it is may be possible to successfully find the component by relaxing the minimum pixel requirement from Setups->Machine Configuration->Parameters -> Setup Variables->Vision->Min Pixels for Flip Chip Center. The table below shows the minimum pixel requirements for representative camera magnifications.

Camera 3 pixels (um) 4 pixels (um)

0.5 mil/pixel 38 51 1.0 mil/pixel 76 102 2.0 mil/pixel 152 203 3.0 mil/pixel 229 305 4.0 mil/pixel 305 406

Programming Sphere or Bump Locations on a Device: Consider the following example device. As stated above, imagine that you are looking through the substrate to see the BGA spheres or flip chip bumps.

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Rules:

• Spheres are programmed as a collection of two dimensional arrays • The location of each array (Xpos, Ypos) is the coordinate of its lower left

sphere relative to the lower left corner of the package. (See above) • The pitch of each array is the center-to-center distance between adjacent

spheres within that array. If there is only one row or column, the pitch is zero.

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Rules of Thumb:

• Program a sphere’s diameter as 75% of the manufacturer’s specified size. In the above example, if the specified sphere diameter is 0.028 inches, the dimension entered is 0.021 inches.

• If the feature is a solder column, program the specified size and use on-axis

lighting. Programming Imaging Information: As it turns out, there really isn’t much to programming light levels for BGA devices. Here are the rules:

• Use side lighting only for all BGAs,C4 , CSPs, or any device that has a sphere or bump that is to be located with the vision system.

• Use On-Axis lighting only for all Ceramic Column Grid Array devices (CCGAs).

• Generally, the default light levels will work well for a wide variety of devices.

The levels should be checked to ensure that only side lighting is entered if side lighting is needed, and only on-axis lighting is entered if on-axis is needed.

Example:

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Programming Algorithm Information:

Centering Type: This selects the processing algorithm that will be used for an area array device. For help in setting this parameter, see the document “Application Notes – Choosing an Area Array Category.doc”. If you are programming a BGA device, set this parameter to “BGA”. Lighting Polarity: The most common setting for this is white. This indicates that the imaged spheres will appear bright on a dark background. Bump Process: The bump process tunes the selected algorithm for a particular application. The following table describes how to set the bump process field. The thing to note about these processes is that each one is a tradeoff. You may be able to optimize one parameter at the expense of another.

Process Description A This process provides general algorithm tuning for a wide variety of devices. Use this as a

starting point. It should work 99% of the time. B This process is used for devices whose spacing between spheres or bumps is smaller than 4

pixels. Tradeoff: Spheres with weak edges may be filtered out as well. C This process is used for devices whose spheres are of low contrast. Tradeoff: noise in the

image is more likely to be found as a sphere, and there should be at least 5 pixels between spheres.

D This processes filters out extra sphere cues. Tradeoff: week sphere cues may also be filtered out.

E This is the same as process A.

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Optional Inspection Information: All Ball Count: Check this if you wish to ensure that all programmed spheres are present on the device. If the software determines that there are possible missing spheres, it will perform additional checks based on the %Missing Ball Threshold parameter. % Missing Ball Threshold: In order to determine a missing ball from a present ball, the missing ball algorithm compares the patch of image where a ball should be to several found ball in its neighborhood. If the information in the patch matches the neighbors to within the % Missing ball threshold, then that ball is said to be present. The higher this value is, the more discriminating the missing ball detection will be. Typically, the default value of 30 is used. Pitch Inspection: Check this box if you wish to ensure that the measured center-to-center distance for rows or columns of a BGA device are to within the tolerance programmed in the % Pitch Tolerance field. If you are using the variable pitch BGA algorithm, this inspection field is changed to the Neighbor Spacing Inspection. The Neighbor Spacing Inspection works as follows:

• The nominal (programmed) center-to-center distance for any bump and its nearest neighbor bumps is compared with the measured center-to-center distance for that same bump pair.

• If the measured distance is Neighbor Spacing Inspection percentage less than the nominal distance, the part is said to be unplaceable. For example, lets assume that the % Pitch Tolerance is the default of 30%. If the nominal distance between two bumps is 200 microns, and the measured distance is 135 microns, the percent difference is ((200 – 135) / 200) * 100% = 32.5%, and therefore unplaceable. Note: If the distance is greater than the programmed center-to-center distance, then no error is flagged. The idea is to catch features that may cause bridging. If the spacing is too large, it is likely that it will fail when compared to another sphere.