-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueIPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
The Cost of Rework
In this issue, we investigate rework's current state of the art. What are the root causes and how are they resolved? What is the financial impact of rework, and is it possible to eliminate it entirely without sacrificing your yields?
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Two-dimensional, Closed-loop Inspection of Stencil Printing
December 31, 1969 |Estimated reading time: 10 minutes
By Robert Kirkpatrick
Traditionally, post-print "paste on pad" inspection has been used to determine if the area of deposition on the board exceeds (or lacks) acceptable limits in either direction. While inspection at this point permits misprinted boards to be removed from the system before component placement, it does not address the underlying issue of preventing the misprint in the first place.
Defining the NeedTo develop an effective method of preprint inspection, experienced stencil-printer operators conducted studies at manufacturing sites to establish the most likely causes of misprints. Other than misalignments between stencil and board, two primary causes of misprints were identified blocked apertures, which result in inadequate depositions, and underside smearing of the stencil, which results in excessive paste deposits.
The studies indicated a need for an inspection system able to perform in two distinct areas: monitoring the stencil underside for blockages and smearing before printing, and checking the top of the board after printing for paste on pads and bridging. Accordingly, a development program was initiated to design a built-in inspection system capable of fulfilling the two-fold requirement.
The operational requirements for such a system include special engineering for high-contrast lighting, software for extended camera vision functions and the ability to produce comprehensive inspection data for analysis. Functional objectives include monitoring screen printing and direct imaging in real-time, verifying critical parameters, maintaining process control and improving yields. In addition, all parameters, settings and limit levels must be fully programmable.
Several months of beta-site testing followed development, which resulted in the introduction of a suite of sophisticated inspection tools providing five modes of inspection capability. They included stencil-aperture blockage, stencil-underside smearing, paste present on pad, paste alignment with pad and bridging between pads. The system* incorporates software that has a "learning" capability and can accept user-defined limits as well as advanced illumination technology. The key element of the system is rapid-response, closed-loop feedback that supports quality assurance and productivity improvement.
Figure 1. Features of the built-in inspection system begin with determining aperture size (a), followed by stencil inspection. Examples include aperture blockage (b) and paste smearing (c). Board inspection includes paste present on pad (d) and presence of bridges (e). Finally, print alignment (paste-to-pad) is reported via X/Y positional error (f).
The Inspection Learning ProcedureAll five inspection modes use aperture size as a datum. Since apertures are intended to permit a quantity of paste to be deposited on pads, they form the basis for the inspection system's learning procedure. Three post-print modes inspect for an area of paste in a known position on the board derived from the size and location of the apertures. Once the apertures are learned, their machine-vision-captured images are overlaid on those of the corresponding pads on the board, a relationship that is also learned. At this stage, the system "knows" what percentage of paste-on-pad coverage represents a 100 percent "perfect" print. The modes operate as shown in Figure 1.
Sites and LimitsThe system uses 4 mm2 active areas or inspection sites. Each site is stored with the board file. More than 200 4 mm2 sites can be defined if required.
Limits of acceptability can be created and applied to individual sites. These define how the printer will respond during the inspection cycle via two levels of response: a warning and an alarm. At the latter, the printer halts. Typical limit settings may be set as shown in the table.
If available, automatic paste dispensing can be triggered by the system when insufficient paste on the pads is detected. Automatic underscreen cleaning cycles can also be triggered when either paste smearing, aperture blockage or paste bridging on the board reaches a limit.
IlluminationLighting technology is the most critical part of any inspection system. To secure robust, reliable data from any vision system, the information must be of the highest quality. A significant challenge is presented by the different surfaces the inspection system may confront.
Figure 2. With conventional lighting (a), uneven HASL surface reflections yield ambiguous illumination. In flat lighting (b), the image is leveled to permit machine vision to function optimally.
For example, hot-air solder-leveled (HASL) boards feature uneven and variable surface contours and reflectivity characteristics. To overcome this, a "flat" lighting system is developed to provide uniform illumination of irregular HASL fiducials and pads via software-controlled intensity adjustment. As shown in Figure 2, fiducials and pads are "leveled" and appear as clearly recognizable shapes, allowing the alignment and inspection algorithms to function to their full potential. The result is a robust alignment that eliminates the need for constant adjustment and increases productivity by reducing operator setup times.
Figure 3. Example of a board printed using the graduated-aperture stencil.
Setting UpInitial qualification of the system is achieved using a special stencil with "graduated" apertures to print a board (Figure 3). The resultant inspection data can be exported to a spreadsheet and used to plot an analysis. The expected trend should be as near as possible to a straight line, indicating the reducing area of paste applied through the graduated-aperture stencil and the system's ability to accurately monitor this process.
Figure 4. Plot uses inspection data to show consistent process control. Each trace represents a different site on the board while the X-axis defines the number of boards passing through the inspection process. The dramatic fall-off after board 13 indicates that the paste ran out during the assessment.
Running in production, a plot showing consistent process control would resemble Figure 4 where each trace represents a different site on the board.
Preprint Inspection for Process ControlThe system's two preprint stencil inspection modes provide process control advantages by delivering meaningful data on print quality and consistency at the earliest process step. The contention that a clean stencil correlates to consistent printing was proven in practice during beta testing of the system where empirical results demonstrated such a correlation.
Using the inspection system as a process control and setup tool, rather than as a constant function of the print cycle, helps balance the need to inspect against productivity goals. The process may also be hastened by using only preprint stencil inspection. Traditionally, print quality was verified by board inspection after printing for adequate paste quantity and pad coverage. Now, the success of the correlation between stencil cleanliness and print quality means that the board need not be checked. If the process is under control, stencil inspection alone can provide a reliable validation.
Another finding of beta-site testing is that inspection is not necessary over the entire area of a board. If critical geometries such as ultra-fine-pitch patterns and micro-BGA** interconnects are "in control," then conventional interconnect patterns should also be within defined acceptance.
Post-print ConfirmationThe post-print paste alignment and paste-on-pad tools provide additional support for adjusting the process and confirming a successful print. A faulty board exiting the printer can easily be washed and returned for reprinting. While the detection of a fault at this stage means that a board has been misprinted, the immediacy of the reporting prevents that board from continuing downstream and subsequent boards from being printed incorrectly.
Closed-loop FeedbackThe key to refining a surface mount assembly process and then maintaining it at optimum performance is a rapid-response inspection system that reports back to the preplacement function. The faster the reporting cycle, the sooner the opportunity is presented for the process to respond.
The inspection procedure can be set to monitor the process using all five modes constantly. As an option, all modes may be used during setup to refine the process, and then disabled during production. The latter option is popular in high-throughput applications where minimum cycle times are required. Naturally, the inspection process can be "re-enabled" at any time. For example, when a new batch is started, printing parameters are changed or product is changed over. The engineering team can also decide whether the system should automatically intervene with a cleaning cycle or simply alert the operator to a detected condition.
One advantage of real-time, closed-loop feedback a dramatic reduction in the need for operator intervention is demonstrated by its immediate influence on stencil printing or direct imaging operation. The printer can be set to produce any of three responses if preset control parameters are exceeded. It can invoke an automatic cleaning cycle, using vacuum to clear a blockage or wet/dry wiping, to remove underside smearing. Or it can issue a warning or alarm, alerting the operator that action must be taken lest the entire process be brought to a stop.
The fail-safe mode, to automatically invoke a cleaning cycle, ensures that the stencil is in optimum condition. However, retaining the option to permit the operator to determine corrective action is important in certain applications. A blockage does not necessarily compromise the next print stroke. Equally, smearing may not affect the electrical integrity of the circuit, although a board with surface patches of extraneous reflowed solder does not present an image of high quality or good process control. The operator's decision to take action is likely to include parameters, such as the characteristics of the paste and the length of time since the last print stroke.
In contrast, if the process is not adequately refined or monitored, there is a risk of passing misprinted boards through to the placement stage of production. When components that add value to the finished product are mounted on a faulty board, extra costs in scrap or rework are incurred. Instead, when process limits are strictly observed and faulty boards are identified quickly, cleaned and returned for re-processing, overall production quality is improved.
ProductivityThe system incorporates a number of automatic procedures to help speed inspection and reduce the impact on cycle time within the production environment:
- A "OnePre" mode for board inspection captures just a single "preimage" of the first board and uses it as a datum for all subsequent paste-measurement inspection cycles. The result is a dramatic reduction in the inspection time.
- Auto-learning of QFP and BGA devices is a technique to speed up the learning routines. Here, only a single feature is learned and the vision system moves around the device to automatically learn the entire component.
- After inspection, the results of print alignment can be directly fed back into the board file to automatically correct the stencil-to-board alignment. This is best achieved in setup mode, not in a normal run cycle.
- In addition, constant monitoring of printer performance can be made using statistical process control (SPC) software tools that generate graphical displays of real-time data to maintain the process in control.
The system also has the capability to inspect critical μBGA patterns for devices down to 0.8 and 0.75 mm pitch. The μBGA routine is also particularly fast owing to the fact that fewer 4 mm2 inspection sites are required to cover the small devices. While the relatively large patterns of a conventional 1.5 mm-pitch BGA chip present no challenge to vision resolution or accuracy, it takes several inspection sites to cover a complete device. In fact, simple math shows that only four I/Os can fit in each site.
In contrast, a 0.8 mm-pitch device can present 16 I/Os in a 4 mm2 site. Thus, just four sites can address a typical device with an array of 48 I/Os in a 6 x 8 matrix, with each site covering 12 I/Os. The full device inspection can be completed in approximately 10 seconds. A large 225-I/O device needs only 16 sites and an inspection time of about 45 seconds.
SummaryReal-time, closed-loop inspection delivers immediate productivity benefits by providing a suite of programmable monitoring tools and user-definable limits and alarms. Technology refinements that add functionality include using the system to determine the suitability of stencil aperture sizes against their corresponding board pads, generating a warning if the two appear mismatched. This level of capability is an inherent function of the system's advanced learning process.
- 2Di (DEK Printing Machines Ltd.)
** micro-BGA is a trademark of Tessera.
ROBERT KIRKPATRICK may be contacted at DEK Printing Machines Ltd., Granby Industrial Estate, Weymouth Dorset, DT4 9TH, UK; (01305) 760760; Fax: (01305) 760123.
Figure 1. Features of the built-in inspection system begin with determining aperture size (a), followed by stencil inspection. Examples include aperture blockage (b) and paste smearing (c). Board inspection includes paste present on pad (d) and presence of bridges (e). Finally, print alignment (paste-to-pad) is reported via X/Y positional error (f).
Figure 2. With conventional lighting (a), uneven HASL surface reflections yield ambiguous illumination. In flat lighting (b), the image is leveled to permit machine vision to function optimally.
Figure 3. Example of a board printed using the graduated-aperture stencil.
Figure 4. Plot uses inspection data to show consistent process control. Each trace represents a different site on the board while the X-axis defines the number of boards passing through the inspection process. The dramatic fall-off after board 13 indicates that the paste ran out during the assessment.