Evaluating the Impact of Powder Size and Stencils on Solder Paste Transfer Efficiency

Estimated reading time: 9 minutes

The solder paste tested included Types 4 and 5 in a modern, no-clean flux medium. The pastes were blended, shipped and stored under recommended conditions. The same metal percentage was used for the T4 (88.5%) and T5 (88.3%) as in earlier tests; however, different lots of solder powder and flux medium were used in this test.

Experimental Design

Input variables in the experiment included:

• Paste Type (4, 5)

• Stencil nanocoating (Y/N)

• Foil mounting tension (std/high)

• Pause time between print tests (0, 30, 60 and 90 minutes)

• PCB pad size (6–15mil)

• PCB pad shape (circular, square, modified square with rounded corners)

• PCB pad definition (NSMD, SMD)

Output variables included:

• Deposit volume

• Deposit height

• Transfer efficiency (% volume). TE is based on theoretical aperture sizes, not measured aperture sizes. All the stencils were cut from the same material on the same cutter by the same operator, so it is assumed that any systemic error is applied equally to all stencils. Actual aperture sizes were not measured

Statistics calculated from the output readings included:

• Means (or averages)

• Standard deviations

• Coefficients of variation (CV, or the standard deviation divided by the mean and expressed as a percentage). CV is preferred over Cpk when comparing different SPI datasets because it normalizes the variation with respect to the mean without the influence of control limits

• Using generally accepted industry practices, the acceptability criterion was TE at least 80% of theoretical aperture volume and a CV of less than 10%

Stencil under wipes were performed before each set of five prints, but not between prints. SPI readings were taken immediately after each print. The full runs that included 0, 30-, 60- and 90-minute pauses took approximately four hours from start to finish.

The tests were nested as follows:

Time 0

1. Install stencil and squeegees

2. Stir solder paste and apply to stencil

3. Print 5 boards

4. Start timer for 30 minutes

5. Remove stencil and squeegees, leave paste on stencil

At 30-minute mark on timer run WVD wipe

1. Print 5 boards

2. Start timer for 60 minutes

3. Remove stencil and squeegees, install stencil and squeegees

4. At 30-minute mark on timer, run WVD

5. Print 5 boards

At 60-minute mark run WVD wipe

1. Print 5 boards

2. Start timer for 90 minutes

3. Remove stencil and squeegees, install “A” stencil and squeegees

4. At 60-minute mark on timer run WVD wipe

5. Print 5 boards

At 90-minute mark on timer "F" run WVD wipe

1. Print 5 boards

2. Start timer for 120 minutes

3. Remove stencil and squeegees, install stencil and squeegees

4. At 90-minute mark on timer, run WVD wipe

5. Print 5 boards

6. Remove stencil and squeegees

Table 2: Data management spreadsheet.

Table 2 shows the data management worksheet that indicates variable, run order, board labeling and SPI file tracking information.

Results and Discussion

There is myriad facets to consider when analyzing the collected data. To streamline the analysis:

1. data has been compartmentalized by component type or padstack

2. only best- and worst-case scenarios are reviewed

3. paste transfer quality is interpreted with respect to the main input variables of nanocoating, paste particle size, and foil tensioning

BGAs

0.4 and 0.5 mm BGA apertures are the first features considered, as these devices are becoming common to the industry and are a frequent print challenge for mainstream PBC assemblers. The test vehicle’s area ratios are 0.71 for 0.5 mm BGA and 0.62 for 0.4 mm BGA. These are at either side of the threshold of recommended guidelines of 0.66 AR, which has historically been cited as the lowest AR that should be considered when printing T3 solder paste. T3 was not included in this test because T4 is quickly becoming the industry standard it is usually readily available. T5 was also tested, as it is often requested by assemblers who perceive smaller powder size as a fast and easy improvement to a fine pitch printing process. T5 paste indeed brings some print advantages, but also carries with it some inherent disadvantages. Procuring T5 paste can present supply chain challenges and it can increase variability in paste print and reflow performance over time due to the higher surface area: volume ratio.

Figures 3 and 4 shows the Transfer Efficiency (TE), or the percentage of theoretical aperture volume that was deposited and the associated Coefficients of Variation (CVs) for the 0.5 mm and 0.4 mm BGAs. Each data point on the 0.5 mm chart represents the average of 3780 deposit readings—84 I/Os per device, 3 devices per board, three boards per panel and five panels per test. Each data point on the 0.4 mm BGA chart represents 16,200 measurements, as the same number of devices have 360 I/Os each. This quantity of data generates high confidence in the results.

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