Selected results from a study on registration using closed-loop control.

As we noted in March 2009, closed-loop process control can be defined as a system that continually monitors and adjusts a process to maintain a particular target value of an output or outputs (Figure 1).¹ To improve the performance of positional accuracy, we tested a closed-loop system that uses a CyberOptics SE300 SPI to measure X, Y and ∆ print offset (Figure 2). This information is then passed to the printer to make appropriate corrections to the print offset, as necessary.




To evaluate the performance of the closed-loop control, three tests were conducted:

• Evaluation.
• Baseline.
• Long run.

Evaluation tests. The objective here was to evaluate the performance of the closed-loop controller interface and acceptable range of correction factor application. Correction factor is defined as the percentage of SPI-measured offset value applied to the next board in the printer. The primary function of the controller is defined here as to adjust the print registration for each squeegee direction until it reaches the target value. This test was conducted using a standard Speedline test board (254 x 203 x 1.575 mm, four-layer FR-4 board with ENIG surface finish). The test vehicle is divided into four quadrants with the same pad layout in each quadrant (Figure 3a). The top half of the board is a “step and repeat,” while the bottom half is the mirror image of the top. Each quadrant incorporates a range of commercially available components and packages (Figure 3b). The print test was run on 40 boards for statistical confidence in the results.

A gauge study on the SPI machine was conducted before the start of the study to ensure the SPI was repeatable. For the gauge study, one board was inspected 15 times (Figure 4). The range for the 15 boards was measured to be less than 5 µm for X, Y offset and 0.002° for ∆. This was considered to be acceptable. Based on these results, the X, Y offset specification was set to be +0.05 mm.

Baseline test. The objective of this test was to baseline the print process without the presence of closed-loop control. The test was performed for 100 boards using a commercially available Type 4 Pb-free paste. The board used was a commercial cellphone board, with four boards to a panel. (Due to the proprietary nature of the product, the actual image of the board is not shown. Instead, a representative schematic is shown in Figure 5.) The board was 191 x 117 x 1 mm with OSP finish. Four areas on the board were chosen to be monitored for X, Y and ∆ conversance, as shown in the enclosed white boxes. At the beginning of the test, an operator using visual methods aligned the board to the stencil. Once optimum alignment was achieved, the process was run for 100 boards without any alignment tweaking.

Long run tests. The objective of this test was to evaluate the stability of the controller for long manufacturing periods (i.e., to simulate a production condition). The test evaluated the ability of the controller to maintain the print process close to the target registration value, and check for rapid convergence. The target for this test was zero offset. The same cellphone board was used for the long run test.
Next month: the results. 

Rita Mohanty, Ph.D., is director advanced development at Speedline Technologies (speedlinetech.com); rmohanty@speedlinetech.com.