Fiducials and Pad Sites

Fiducials and Pad Sites

A fiducial is a board feature used for global and local error correction to determine the difference between programmed coordinates and actual locations on the board. This ensures that parts are not placed before their locations are verified.

 

A pad site is a pad pattern on the production board that can be used in the same manner as a fiducial.

 

The most typical types of fiducial failures are caused by improper color, size of fiducial, and lighting values. Other factors such as the confidence level and search area can also be trouble spots but as the programmer’s experience level increases, these will be less likely to cause problems.

 

How many fiducials to use on a board or circuit will depend on board quality and the amount of time the manufacturing process can allocate to finding fiducials. The following is a general guide as to the number of fiducials used and the benefits of accuracy.

 

 

Number of Fiducials Found

Correction Possibilities

 

1

 

X and Y

 

2

X, Y, and Theta

 

3

 

X, Y, Theta, and Uniform Stretch

 

4-5

X, Y, Theta, and Independent X & Y Stretches

6-10 (max)

 

X, Y, Theta, Independent X & Y Stretches, and Corners not equal to

90°

 

The total number of fiducials and pad sites that can be used for a global correction cannot exceed ten.

 

To use a combination of fiducials and pad sites for global error correction, you must assign them in the Circuit List window.

 

The total number of fiducials and pad sites that can be used for a local correction cannot exceed five.

 

To use a combination of fiducials and pad sites for local error correction, you must assign them in the Local Fiducials dialog box in the Placement List window.

 

When creating a fiducial or pad site, use the Tab key to move between the data fields. If you use the Enter key, the fiducial placement is attempted and error checking is performed.

 

To successfully create valid fiducial placements:

–      Fiducials must be placed within the borders of the board.

–      Fiducials cannot be placed directly on offsets. (Fiducials placed on circuits

are automatically duplicated on all the offsets associated with that circuit.)

–      Fiducials cannot be partially on a board or circuit.

 

If a fiducial is on an offset and that offset is rotated, the fiducial location is rotated but the fiducial is not. Only fiducials with rotational symmetry are supported in this manner. All others will not be found.

 

If multiple fiducial or pad site definitions are selected when using the Fiducial or Pad Site Copy function, all new fiducials and pad sites are distanced from the originals by the same X and Y Offset values.

 

If fiducials or pad sites are consistently not found by the vision system, lower the confidence level. If the vision system finds objects other than the fiducials or pad sites, increase the confidence level.

 

When defining a search area, keep in mind that it should be large enough to allow some tolerance in board handling, but not so large that additional board features are found instead of the fiducial or pad.

 

Some recommended lighting levels for fiducials and pad sites.

 

Fiducial Type / Pad Site

Inner Ring

Outer Ring

Tinned / Tinned

80 / 80

20 / 20

Solder Mask over Bare Copper (not recommended) / Gold

0 / 0

50 / 35

Bare Copper with Copper Bright / Bare Copper

0 / 0

35 / 35

 

 

The pad site functionality is not available for the Odd Form system at this time.

 

In most cases, standard lighting cannot be used to image a pad site since solder paste or flux may not allow a good contrast between the pad site and the circuit board. Special lighting settings may need to be installed in order to image the pad site. If  Pad Site Find is the only way to get component corrections, and lighting is the only issue, consult your UIC Application Engineer.

 

Use the Fiducial Lighting procedure located in the Operation Features Module within the User’s Guide, to determine whether a pad site can be imaged with the PEC camera. Verify contrast and the lighting level required.

 

When to use Pad Site Find

1) When fiducials do not exist on the circuit board

2) When the pad site accurately represents a component type

3) When fiducials do not give an accurate enough correction

4) When accuracy is more important than speed

 

If any errors occur finding pad sites, you will be taken to the Fiducial Repair screen. In the case of failed pad site finds, manual alignment is not recommended. For GSM1 systems, select the Reject Board button to remove the board. For GSM2 systems, palm down the machine to manually remove the board.

 

The need for a pad site correction is more typical of fine pitch placements such as C4 placements or fine pitch BGA’s.

 

Pad sites are based on component definitions. To associate a pad site definition with a component, the component must be defined in the database. Refer to the New Component module for information on adding a component to the database.

 

 

PEC Lighting

 

On the GSM machine, a Pattern Error Correction (PEC) camera passes an image to the vision system which attempts to recognize a programmed fiducial or pad site based on parameters in the Fiducial or Pad Site List. These parameters consist of type and size, center of fiducial identified by its “X,Y coordinates”, and the search area identified by “Search Area X,Y”.

 

After the PEC camera moves to the programmed location of the fiducial, it illuminates the Search Area using the programmed “IN/OUT” (inner ring/outer ring) light levels. Within the search area of the image, light intensity differences between the fiducial and the board help the vision system detect the fiducial’s edges.

 

The vision system is able to detect the North, South, East, and West edges of the fiducials by relying on the differences in contrast between the board and the fiducial color. Called vector points, triangles of red, blue, green, and yellow are displayed in the Vision Window.

The vision system uses six vector points per edge (N, S, E,W). In order for the vision system to obtain 100% confidence, 24 out of 24 of the vector points must be detected on an edge of a fiducial. The default confidence level is 80% (19.2 rounded up to 20 vector points).

 

Since the success of fiducial finds depends on the vision system’s ability to discern the contrast between the board and the fiducial, some combinations of fiducials (or object(s) to find) and their backgrounds may call for different types of PEC cameras. Currently 2-sided and 4-sided lighting is being used and FlexLight, a new feature, will soon be available. The 2-sided PEC camera was non-symmetrical in its lighting pattern. It illuminated in one direction, from the North and South. The 4-sided PEC camera improved on this by illuminating in four directions, from the North, South, East, and West. Originally both cameras used red LED’s.  When looking at solder-mask covered fiducials, the red light would be absorbed by the solder mask (green). To overcome this problem, green LED’s were added. The 4-sided scheme expanded the capability to illuminate gold fiducials on white ceramic as well as fiducials on flexible circuits.

 

FlexLight (trademark) is an enhanced PEC lighting module.  It was originally developed to address the imaging challenges associated with advanced substrates such as ceramics and flexible circuits.  Although FlexLight was initially targeted at these markets, it can effectively image a wide variety of substrate materials ranging from FR-4 to more exotic materials.  The chief advantages of FlexLight are: 1) Symmetric illumination, 2) Polarization flexibilty,

3) Wavelength flexibility, 4) Ease of reconfiguration, and 5) Monolithic design.

 

A mechanical support structure holds eight LED petals and an inner LED ring. Each petal is a small printed circuit board containing 10 LED’s.  The petals can contain light sources of various wavelengths ranging from blue to red.  The petals and the inner ring can be exchanged in a “plug-and-play” fashion.  This allows the illumination wavelengths of the module to be quickly and easily changed.  It also facilitates ease of service in the field. The supporting electronics allow the petals to be configured in various series and parallel combinations to support a wide variety of LED’s.

 

The structure supports an optional polarizing film that covers four of the eight petals as shown in the following diagram.

Corner Feature Enhancement for Multipattern Components

 

Multipattern components consist of components or objects (RF shields, connectors etc.) which cannot be described adequately as either leaded or leadless components, but rather are defined in terms of an arrangement of geometric features.  The multipattern object is located by locating each of the features of which it is comprised, using a single or multiple fields of view.  One such feature, which is commonly used to locate rectangular or pseudo-rectangular objects, is the corner feature.  At present, this feature is defined simply by entering the length of each of the two line segments, which make up the 90 degree corner (the horizontal corner edge length and the vertical corner edge length).  With this special software, this feature definition has been extended to allow for two more optional parameters.  These parameters define “ignore zones” at the apex of the corner, and allow the image processing to ignore these regions of the edges when locating the corner.  By this means corners which are rounded, chamfered or poorly defined at the apex can still be located by using segments of the corner away from the apex, which subtend 90 degrees to each other.

 

The diagram below indicates the meaning of each of the parameters.

 

 

 

 

 

X2, Y2 should not exceed 25% of X1, Y1

If  X2 or Y2 = 0, the standard corner find is employed

 

 

 

 

 

Enhanced Product Setup

 

 

A very helpful feature when programming components is Enhanced Product Setup. It consists of two parts, Enhanced Component Setup and Enhanced Board Setup. Each process involves a live image, of the object being taught, to be manipulated while the programmer sees the changes as they are being made.

 

When defining a new component, fill in as many data fields as possible while paying special attention to the following; Component Height, PreOrient, Number of Leads, Lighting Type, Camera Type, Default Feeder, Default Orientation, and Reject Station.

 

Enhanced Component Setup supports, Four Spindle, C4, OFA (Oddform Assembly) and High Accuracy (UFP) Heads.

 

If anything goes wrong with the Platform machine during this entire process (reject station not mounted, feeder not mounted, exclusion zone, drop bin not defined, centering fails due to invalid parameter, etc…) recover by palming the machine down, and up again. Then push the Start button and proceed to pick the part again.

 

If the Platform machine was not calibrated correctly prior to using EPS, the scale of the drawing may be incorrect and the Draw Component function cannot be used.

 

All changes made are immediately written back to the database scroll list where the part was defined. Exit the Inspection screen at any time to view the results of the changes there. Nothing is saved permanently until the part is saved.

 

 

Common ECS Hinderances and Solutions

 

Before the part can be picked, all the values associated with component definition must be entered. This is necessary because these values are all needed to inspect a component.

 

All changes to the drawing are immediately applied to the definition database of the component. If a mistake is made, rectify the error by using the Undo function. No change is permanent until the component is saved.

 

To switch from editing the body of the drawing to any of the leads/bumps/features, click on the leads/bumps/features. To switch back to editing the body, click where there is no lead/bump/feature.

 

Due to the method used for programming leads, it can be difficult to line up all the leads over their displayed counterparts. This is because pitches are measured from the center of the side of the component, and when they are adjusted, leads move symmetrically out or in from/to the center. To help the adjustment, if there is an odd number of leads, position the single lead in the center of a side over its corresponding displayed counterpart. If there is an even number of leads, position the two center leads over their displayed counterparts before adjusting the pitch.

 

To define a C4 component it is sometimes convenient to define only one bump initially, and add bumps when the image is displayed, wherever necessary until the part is found. This is a good procedure because it may be difficult to determine how bumps will image before seeing an image of the part.

 

When dealing with a large number of leads/bumps at once (over 50), the drawing function will automatically move only the single lead selected, instead of all the leads. This is done to increase the performance of the drawing operations. If less than 50 leads/bumps are selected, they will all be repositioned at once to give a better indication of their final positions.

 

One of the more difficult things to deal with is when the displayed part’s rotation is slightly off. Make sure that the feeder pick position is optimal to present the part accurately. Use the pick/inspect/drop-off sequence more than once if necessary until the part is basically square on the screen.

 

Lead groups can cause additional problems. The drawing always assumes that all leads are present on a side, but does not draw some of them if they were deselected in the leadgroup screen. This can make it difficult for pitches to be adjusted.

 

If the component is too large to fit into a single field of view, the vision system will take more than one image and stop at the first image where it could find all leads/bumps/features. This might be the first image seen, or the last. If the part is found successfully, it will be the last. This makes editing of the components, by using the Draw Component function, difficult. Sometimes it is more convenient in this case to go back and forth between the Database Component Definition screen and the Inspection screen.

 

When viewing a component on the monitor, the image detail may require enhancement. With the use of Vision Level Diagnostics, the operator can increase or decrease the detail of the viewed image by raising or lowering the current vision level. By increasing the Vision Level Diagnostics to a level 5 setting, the operator can view the image with the maximum amount of detail. Using a lower vision level results in a decrease in display detail.

 

 

 

Specific Component Programming

 

If a change is necessary while adding a  new component to the database, do not change the component type, exit and begin the procedure again.

 

The Accuracy field applies only to a GSM2 (Dual Beam) machine. When the value is set at high, this means stop the opposite beam while I place this particular part with the other beam. Our accuracy studies indicate there is no need to ever run the machine with this value set to high. It adversely effects throughput and does not contribute to the accuracy of the machine when placing standard SM devices. Ignore this field for any other machine configuration.

 

For parts that do require a more accurate placement it may be advantageous to turn on preorient.  This indicates to the machine that the part will be rotated to it’s place rotation prior to being scanned through the upward looking camera.This allows the machine to minimize the amount of correction required after being centered and inherently contributes to a more accurate and repeatable placement.  It does however adversely affect throughtput.  Therefore, if you find you the placement accuracy does not meet your expectation with preorient turned off, turn it on and reevaluate the accuracy/repeatability of your placements.

 

When choosing a lighting level for BGA, C4, or C4-Pattern components, a level of +7 should only be used with side-lighting.

 

 

C4 Types

 

The following restriction applies to programming C4 components on a machine equipped with an AISI 3500 vision system: A maximum of 16 unique C4 components, with 20 programmed features per component, can be contained in a product. This restriction is based on the number and type of programmed C4 features.

 

Placement pressure values above 350 grams are typically used for C4 applications. If the placement head is not C4 capable, these pressures will not be possible.

 

The current bump process is ‘A’, selected as the default. Bump processes B-E are reserved for future UIC vision inspection algorithms.

 

The X or Y Vector value will be ignored if the X or Y Number value equals 1.

 

The % Bumps Required for a C4 component is the percentage of bumps required to return an accurate image.

 

If C4-Pattern is not available from the Component Types list box, you must create a new database. This is done by using the New option under the Database menu bar heading. If desired, existing component definitions can then be brought into the new database using the Merge option.

 

For C4-Pattern, the value for Critical should be chosen as Yes.

 

There should be no entry in the Min Precise Patterns, Pattern Inspection, Location Tolerance X, Location Tolerance Y, or Relative Distance fields.

 

BGA  Types (Requirements and Limitations)

 

A special version of software is needed, developed after an RFQ, for use with UPS 2.x

 

The component can only be processed in a single field of view

 

The appropriate magnification, circular lit camera (circular lit cameras take up 2 additional feeder slots

 

The vision system must be an AIS630 Lantern vision system only.

 

The % Bumps Required for a BGA component is the percentage of bumps required simply to display an image.

 

Missing Ball detection for BGA components

 

Centering – the vision system identifies the defined features (bumps) and determines the x, y, and theta corrections required for an accurate placement. Bump Process A should be chosen in the component definition.

 

Inspection – after the centering process is complete, an additional algorithm is applied to determine if any bumps are missing. When centering and inspection are is desired, Bump Process E should be chosen in the component definition.

This software inspects BGAs for missing balls using a two step approach.  First the regular ball find algorithm is executed and five candidates are selected as potential missing ball sites.  The selection is based on either the failure to locate a ball at an expected site, or a low correlation, or ball recognition score.  Then an intelligent pattern recognition algorithm is trained  on sites which are known to contain good ball images, and the trained algorithm is used to classify the suspect sites and verify the presence/absence of a solder ball.  Various graphic overlays are used during the execution of the algorithm:

 

  • It will be necessary to use circular lighting for bump imaging in order to realize optimum reliability. This is because the image quality of balls with the standard lighting is poor.
  • This algorithm uses a training method based on balls which are found.  If the image quality is such that noise can be incorrectly labeled as a ball, it is possible to mis-train the algorithm and fail to correctly identify missing balls.
  • Only components which fit into a single field of view can be processed.
  • In order to switch on missing ball inspection the customer must select “processing type E” in the product editor (the default is A).  This processing type flag is provided to allow for customer defined image processing and in general is not used.  It is expected that using this flag will have no impact on the overall functionality of the machine, since processing types B-D are still available for customer specific tuning.
  • This will be a special vision release to support the missing ball inspection.
  • The five missing ball candidates are labeled by blue crosses with blue boxes.
  • The trained existing balls around the missing ball candidates are labeled by blue crosses only
  • The recognized missing ball is labeled by a small red cross on the center of the candidate label

 

If the colored graphics are an annoyance, you can change the Vision Diagnostic Level. The value is probably set at 4 or 5. The range is between 0-5. The lower the value the faster the machine.

 

BGA Type

1.4x UPS

Pick and Place

Capable

2.x UPS

Pick and Place

Capable

Special Camera

Requirements

for inspection

Missing Ball

Inspection

Capability

CBGA (ceramic)

Yes

Yes

None

Need Analysis

CCGA, White (ceramic-column)

Yes

Yes

None

No

CCGA, Dark (ceramic-column)

Yes

Yes

None

No

uBGA

Yes

Yes

2.6-3.0 Mil/Pixel Camera

Need Analysis

PBGA (plastic)

Yes

Yes

None

Yes

TBGA (taped)

Yes

Yes

Circular Lighting

No

 

Camera

Maximum Single Field of View Size

Minimum Pitch

Minimum Ball Diameter

Super High Mag (0.5 mil/pixel)

4mm (0.160”)

0.125mm (0.005”)

0.075mm (0.003”)

High Mag

(1.0 mil/pixel)

10mm (0.39”)

0.25mm (0.010”)

0.125mm (0.005”)

Medium Mag

(2.6 mil/pixel)

20.8mm (0.8”)

0.5mm (0.20”)

0.25mm (0.010”)

Medium Mag

(3.0 mil/pixel)

24mm (0.8”)

0.5mm (0.20”)

0.25mm (0.010”)

Standard Mag

(4.0 mil/pixel)

32mm (1.25”)

0.8mm (0.031”)

0.4mm (0.016”)

 

Leaded Components

 

Lead information must be programmed symmetrically. Information entered for Sides 1 and 2 of the component is input to Sides 3 and 4, respectively. The data can then be edited. To accommodate nonsymmetrical components or components with different lead lengths or pitches, the Lead Groups option may be used.

 

Lead groups can cause additional problems. The drawing always assumes that all leads are present on a side, but does not draw some of them if they were deselected in the leadgroup screen. This can make it difficult for pitches to be adjusted.

 

If 0.0 (zero) is entered in any of the following Tolerance data fields, that inspection is bypassed; Lead Tolerance From Body, Lead Tolerance Across Body, Lead Spacing Tolerance, Lead Length Positive Tolerance, Lead Length Negative Tolerance, Coplanarity Tolerance, and Colinearity Tolerance.

 

If an excessive number of components are rejected, check the component definition relative to vendor specification sheet for the component. Also, use ECS (Enhanced Component Setup) to adjust inspection parameters (geometry, lighting, etc…).

 

Lead Groups

 

The Lead Groups window is not used to toggle leads off for the purpose of increasing the speed of vision inspection (SMC components only). This will only result in a rejected component. All components must be defined as they physically exist. Non-symmetrical leads can be accommodated by defining the component as a Special-Leaded Component.

 

Lead 1 in the component database is not necessarily the component’s electrical pin 1. It is only the first lead in the lower left corner of the component when the component is in the 0° orientation. We define/assign leads as beginning with lead one in the lower left hand corner and count up as we define the part in a counter-clockwise fashion.

 

If you select the Remove All Leads option, all component leads are toggled off and considered to be phantom leads. If a lead was already toggled off when the Remove All Leads option was selected, it would remain off.

 

If you select the Enable All Leads option, all component leads are toggled on and are inspected by the vision system. If a lead was already toggled on when Enable All Leads option was selected, it would remain on.

 

Special Leaded Components

 

Program the component as if all leads on the same side are identical and symmetrical with each other.

When defining a component with different pitches, find the greatest common denominator and enter that as the pitch.

 

The machine memory supports a maximum of 15 lead groups per component.

 

When all lead information is entered, select the Lead Groups option. Select the leads you want to be ignored by the vision system. The leads are now phantomed with just a broken line to indicate their existence.

 

Example:

Let’s use the 23pin SMT connector as an example…  There are physically 12 leads on one side of the device and 11 on the opposite side. It would be a reasonable approach to define both sides as having 23 leads with a pitch of 1mm, and turning off every other lead in a manner where the database matches the physical description of the part. However, by turning off every other lead this creates 23 lead groups, and this is why the machine hangs up!

 

We define/assign leads as beginning with lead one in the lower left hand corner and count up as we define the part in a counter-clockwise fashion.  For example, for a 14 pin SOIC, lead # 1 is in the lower left corner and lead # 14 is in the upper left corner (assuming the part is defined with the leads facing north and south).  There are two lead groups when we define a 14 pin SOIC.  Lead group 1 is defined as leads 1-7 and lead group 2 is defined as leads 8-14.  However, if you turn off lead 4 there are now 3 lead groups (lead group 1 = leads 1-3, lead group 2 = leads 5-7, and lead group 3 = leads 8-14).  Notice lead 4 is not included.

 

By turning off every other lead you are creating 23 lead groups. We only have enough RAM on the machine controller to support a maximum of 15 lead groups. However, the number of lead groups is dynamic and can be limited (reduced) by the number of components, component placements, and process complexity.  Therefore, the number of supported lead groups can be £ 15, depending on the product complexity.

 

Program the part as it is…  Assuming the part is coming in tape and the12 leads are facing 6 O’clock and the 11 leads are facing 12 O’clock, let’s define the part as having 12 leads on side 1 at a pitch of 2mm and side 3 as having 11 leads at a pitch of 2mm.

 

 

 

 

 

 

 

 

 

 

 

Component Terminology

 

Acronym            Name

 

BGA                   – Ball Grid Array

uBGA                 – micro Ball Grid Array

CBGA                 – Column Ball Grid Array

C4 or Flip Chip   – Controlled Collapse Chip Connection

COB                    – Chip On Board

CSP                     – Chip Scale Package

DCA                   – Direct Chip Attach

FPT                     – Fine Pitch Technology (20 to 40 mil pitch)

ILB                     – Inner Lead Bonding

MCM                  – Multi Chip Module

MELF                 – Metalized ELectrode Face bonded

MSP                    – Mini Square Pack

OLB                    – Outer Lead Bonding

OMPAC              – Over Molded Plastic pad Array Carrier

PBGA                 – Plastic Ball Grid Array

PLCC                  – Plastic Leaded Chip Carrier

PQFP                  – Plastic Quad Flat Package

QFP                     – Quad Flat Package

SOD                    – Small Outline Device

SOIC                   – Small Outline Integrated Circuit

SOJ                     – Small Outline J lead

SOT                     – Small Outline Transistor

SQFP                  – Shrink Quad Flat Package; QFP with a lead pitch of .016” or less

TAB                    – Tape Automated Bonding

TSOP                  – Thin Small Outline Package

UFPT                  – Ulta Fine Pitch Technology (<20 mil pitch)

V-QFP                – Very Small Quad Flat Package

V-SOP                – Very Small Outline Package

 

 

 

Industry Terms

 

CER-QUAD              – Digital Equipment Component

C-QUAD            – Northern Telecom Package

Tape Pak             – Trade Mark/National Semiconductor

V-PAK         – Vertical Package (Texas Instruments – memory package)

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