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Expanding Role of Machine Capability Analysis
December 31, 1969 |Estimated reading time: 8 minutes
When considering the purchase of an expensive piece of manufacturing equipment, it is important to know that the equipment will perform to spec. With much consideration given to ROI, enhancing profit margins, and predictable payback of equipment, users must be sure that equipment will fulfill process requirements. Verifying a machine’s performance beforehand is key.
By Michael Sivigny
It is important to know that a new equipment purchase will perform precisely to the manufacturer’s original specifications. This may seem obvious, but often the buyer will take the seller’s word that a piece of equipment is performing to spec. Once the equipment is on the shop floor, however, there are problems. Faith is a poor gauge of equipment performance. With so much consideration given to return on investment (ROI), enhancing slim profit margins, and predictable payback of equipment, it’s important to be certain that the equipment under consideration, whether it is a printer, a dispenser, a pick-and-place machine, or a reflow oven, will fulfill process requirements and meet the demands of its intended use. One way to be certain is to verify the machine’s performance beforehand through machine capability analysis (MCA). This applies to new and used equipment. The scope of machines to which MCA testing can be applied is growing, including SMT assembly equipment, routers, and laser-marking machines. This list is also expanding into semiconductor processing.
A seller cannot always provide this verification. With third-party brokers of used equipment, for example, they may not have the equipment, training, or capability. With new equipment suppliers, such testing and verification is best performed by an objective third party, such as a mobile testing service that can look objectively at the machine’s performance and provide usable information about what the purchaser is buying.
It may be unfair to say that one should not always trust information or assurances from the supplier. The supplier may have the best of intentions, basing their assurances of machine performance on in-house studies or knowledge. However, there are many instances where the machine being purchased may not have been evaluated properly, so it is important to have a machine tested prior to purchase to verify performance and correct any offsets or inaccuracies. This will ensure performance according to the original manufacturer’s specifications on the shop floor.
Is this type of testing unnecessary with a new pick-and-place machine? How would a reputable supplier of such equipment respond to requests for performance verification? When a customer asks the supplier for performance verification, OEMs generally are willing to answer those questions. The manufacturer may or may not be able to provide such information, depending on whether or not they have appropriate measurement gear on site. If not, they will more than likely bring in a reputable third-party company to perform the necessary machine analysis or verification. The manufacturer will find a solution to accommodate the customer.
How Does MCA Testing Work?
The purpose of performing capability measurement is to validate specification compliance. This means making sure that the machine meets intended quality specifications. Typically, a testing company will go into the facility where a machine is located with a field-test unit. They will perform the evaluation and give the customer a third-party objective report. The customer will decide what to do with the results. Sometimes, the testing company can correct offsets in the machine. This may require a service call from the equipment’s original manufacturer. The goal, in any case, is to provide the customer with a solution.
Testing placement accuracy uses actual components in the product assembly. The pick-and-place machine places chip components on a test fixture - an accurate glass plate marked with fiducials that is analyzed against Gerber data. Measurements are taken for Cp and Cpk indices. Software operates measurement equipment and provides statistical specification-based results on machine-quality performance (Figure 1). Comprehensive certification reports validate performance, allowing users to improve product quality and optimize performance for increased manufacturing profitability.
Figure 1. An “across-the-board” statistical evaluation (circled in red) of placement equipment yields unacceptable quality performance. Data shown indicate calibration and/or head and angle offset adjustment is needed for further machine optimization. Across-the-board and board-to-board (repeatable) performance data must meet manufacturers’ specification criteria.
In a chip shooter, for example, common problems are systematic offsets, which can be per head, per nozzle, per angle, or a general offset. Offsets affect chip-shooter accuracy negatively, resulting in PCB defects, lower yields, and costly manual rework. With growing demand of 0201 and 01005 chip use, placement accuracy is critical (Figure 2). MCA looks at basic settings and functions of the equipment (clamping, sensors, nozzles, camera, feeders, etc.) to identify, control, and correct failures so the chip shooter can assemble a product within the manufacturer’s original quality specifications. If the chip shooter requires adjustment, the testing company may provide the fix, or the customer may contact the equipment manufacturer to schedule a service call. Getting the chip shooter back to optimal operation will ensure best-possible yields and low defect rates. This testing also can be applied to dispensing, printing, and reflow machines.
Figure 2. A highly accurate glass plate, camera, and relative measurement methodology allow for capability analysis of placed chips and live QFP devices. Attribute quality information is collected and reviewed for machine performance, and can be fed back to component suppliers for further quality improvements.
The transition to lead-free processing has made the need for MCA testing more critical. Lead-free solders do not wet as readily as traditional tin/lead solders, making printing and placement accuracy more critical. In an instance where SMT pads are not printed accurately, for example, where they are printed partly onto a non-solderable area or mask, it is likely that the solder will not wet the entire pad. Similarly, a lower surface tension in the liquidus solder may inhibit auto-centering, which assemblers count on to center slightly offset components in tin/lead soldering. Keeping the lead-free process window wide requires that assembly equipment operate optimally at its original performance specifications. This keeps the process in control and helps ensure minimal defect levels and maximum yields.
Similarly, MCA testing is applied to automated dispense systems. This is important as the size of SMT passive components shrink, making dispense accuracy for adhesives and solder paste increasingly critical. By running a capability test, basic dispenser settings and functions are checked (clamping, sensors, nozzles, camera, robot, etc.). Dots of material are dispensed onto an accurate patterned glass test plate and evaluated against a known-good Gerber data file - measuring positional accuracy of the dots. The glass plate is used to minimize dimensional errors associated with an FR-4 board, providing accurate and relevant measurements. The test measures the positioning accuracy (X, Y) of the dispense gantry and evaluates deposit accuracy and diameter consistency. Measurements take into consideration the impact of placement order, head type, single dot vs. double dot, needle configuration, and other contributing parameters. With adhesive dispensers, it is important to dispense glue dots with a proper shape and in the correct locations. The most common problem found on a dispenser is mean-value offset. This can be an offset per head, from incorrect camera-to-needle calibration, or from a gantry offset. When the problem stems from a systematic deviation, test data must be used to calibrate the machine. Mean values associated with heads and gantries can be applied to the machine as offset adjustments, which further optimize dispensing performance (Figure 3).
Figure 3. Accurate glass plate and components are used to remove variations found in production materials. The accuracy of materials and measurement systems are critical to evaluate and optimize SMT and semiconductor production equipment.
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Working with SPCs
Test or measurement results can be grouped statistically and used to further evaluate or adjust the machine. Typically, when using statistical process control (SPC) methodology in the production line, one modifies overall line performance and uses specification limits for the overall process. Within specifications limits of the overall process, one has individual machine specification limits that contribute to the performance of the overall line goals. Therefore, when one observes a deficiency in certain areas of the manufacturing line, or a machine that is creating some out-of-specification problems, the ideal scenario would be to conduct an MCA test on that particular machine, find the root cause of the problem, correct it, and put the machine back into production. You also should monitor the overall process goals for the line carefully.
Central Reference Point for the Industry
A reason for independent third-party verification of machine performance specification is that there are no standard criteria for the qualifying factors used in determining equipment performance specifications, or what specifications should be supplied to the customer.
Third-party testing provides standard, statistically valid, well-documented data. Equipment performance specifications that are verified using a valid third party minimizes the specification investigation process, and generates confidence in published specifications. MCA testing can help manufacturers comply with industry standards, where such standards may be designed for machine-to-machine comparison and not necessarily process optimization. One example of this is IPC-9850, which standardizes parameters, measurement procedures, and methodologies used for the specification, evaluation, and verification of assembly equipment characterization parameters. The ANSI-approved standard also establishes procedures to characterize and document machine placement capability of surface mount assembly equipment, while maintaining a placement-accuracy-to-placement-speed relationship. However, IPC-9850 is more a machine-to-machine characterization comparison, and not designed for performance optimization. A properly executed MCA is a true indicator of performance capability, and a suitable tool for an equipment owner seeking compliance with IPC-9850. The third-party testing company may be able to perform IPC-9850 characterization on all manufacturers of placement equipment in the user’s facility - focusing on accuracy and repeatability.
Conclusion
A machine that is not performing to design specifications likely is causing defects. Defect levels rise significantly as machines operate further outside of their published specifications. Increased productivity and reduced defect levels can be realized with regular use of machine capability measurements. Quality-certified machines, with documented reports for ISO and QS9000 compliance, allow users to improve product quality and optimize performance for increased manufacturing profitability.
Michael Sivigny, general manager, CeTaQ Americas, may be contacted at (603) 883-7843; e-mail: msivigny@cetaq-americas.com.