Advancement of SPI Tools to Support Industry 4.0 and Package Scaling

Estimated reading time: 13 minutes

Measurement Fundamentals

Accurately measuring solder paste deposits’ height and volume depends on several factors, such as the imaging sensor’s resolution, PCB surface undulations, height threshold settings, the PCB’s optical characteristics, and the post-processing algorithm that computes height and volume. A recent paper ^[6] describes in detail all of these in addition to proposing a better alternative procedure for measuring solder paste volume.

Figure 4: Design image of golden reference board (red: BGA pads, blue: QFP pads, green: passives).

SPI Basics

Measuring solder paste deposit accurately is not an easy task. First, the PCB surface is not smooth; it can have local warpage, undulations on surface, etc. Second, the solder paste deposit location on the metal pad is a mix of three intersecting surfaces: metal pad, solder Mask, and FR-4 PCB material. This complicates identifying and setting up a reference plane for measurement. Third, the solder paste deposit itself is not a perfectly defined cylinder or cube shape. SPI tools use white light and/or laser light to measure deposits. A phase-shift, moiré interferometry technique is widely used. White light from projectors is passed through gratings to form interference fringes. It is shone upon the uneven surface of the solder paste deposit, and the image is captured at known angles.

The shift in the projected interference image is measured using a CCD camera, and basic triangulation mathematical techniques are used to calculate the height of the solder paste deposit. The pad area of the solder paste deposit is broken down into pixels, and the height of each pixel is determined. The size of each pixel depends on the camera resolution being used. Individual pixel measurement is statistically evaluated to obtain the height of the deposit on a particular pad ^[2]. To overcome shadow problems, certain vendors use dual projection or more for uniform illumination of the deposit. Besides moiré interferometry, laser triangulation is also used by certain vendors to measure the Z-height of the PCB pad/solder paste deposit.

The exact height of the metal pad with respect to a reference plane is difficult to determine. Most SPI vendors end up setting thresholds (the industry specification is around 40 microns) below which measurement is ignored. For more accurate and slower measurement route, the bare PCB is run through the SPI tool first before printing. PCB warpage, surface morphology, and pad height are captured and taught. Next, a pasted PCB is run, and the bare PCB pad height is subtracted from the solder paste height calculation.

GT Measurement Basics

The GT is a non-contact profilometer with an X-Y motion system. It uses a chromatic white light sensor. In brief, the working principle is as follows. Polychromatic white light composed of several wavelengths is focused on the object surface. It is well known that different wavelengths have different focal lengths. For any point on the sample surface, one reflected wavelength component of the white light will have the maximum intensity relative to all other wavelengths. The spectrum of reflected light is analyzed using a spectrometer. If a particular wavelength is exactly focused on the target, the spectrum has a maximum at this wavelength and shows a peak. To each wavelength, a specific distance to the target is assigned by calibration. Therefore, by knowing the wavelength with the highest intensity, it is possible to accurately map the height profile point by point over the entire sample surface.

Figures 5a and b: Variability chart for bias % for Phase 1 volume range.

Key differences are to be noted between the inline SPI tools and GT. The GT is a lab-scale, metrology-grade tool with a focus on accuracy and resolution vs. throughput. Its height and depth resolution is ~160 nm, and spatial resolution could be down to 1 micron. The GT runs much slower than SPI tools. Inline SPI tools are automated tools with a focus on speed and throughput. These tools are gross inspection tools. The GT and SPI machine use completely different optics and technologies to perform the measurements.

Results and Discussion

The GB was measured on the GT and then sent to various SPI vendors (A through E) for measurement. All of the vendors were provided the same board and stencil Gerber data for recipe creation and measurement. Accuracy bias in volume % measured was calculated and compared using the following formula: accuracy bias % = (Vol GT – Vol X)*100/Vol GT, where X denotes different SPI vendors. All of the SPI tools evaluated in this study are based on shadow-free moiré technology. Camera resolution is different for each vendor, as noted in Table 2. Besides camera resolution, there are other hardware and compute algorithm differences among SPI vendors A through E. Select pads from BGA, QFP, and passive components were used for accuracy bias measurement.

Solder paste volume of these components is in the range of 0–900 cubic mils. Solder paste volume ranges were divided into three phases—Phase 1: 0–100 cubic mils, Phase 2: 101–250 cubic mils, and Phase 3: 251–900 cubic mils. Allowable accuracy bias variation criteria were kept at ±20%. Accuracy bias for Phase 1 volume range is plotted in Figures 5a and 5b. We can clearly observe that for very small paste volume deposits between 0–50 cubic mils for passive and BGA components (Figure 5a), all of the vendors except A+ show a higher bias variation than 20%. Tool A+ is a newer version with a much higher resolution camera and software upgrades from vendor A. Also, the measurement is performed without any threshold setting on the A+ tool. Tools A and B perform worse for small-volume measurements with a huge accuracy bias %. For the 51–100-cubic-mil solder paste volume range shown in Figure 5b, only tools A+ and C meet the criteria; the rest are failing either for a positive or a negative bias percentage.

Figures 6a, b, and c: Variability chart for bias % for Phase 2 volume range.

Phase 2 solder paste volume ranges are further broken down in three subparts for analysis, as shown in Figures 6a, b, and c. Part 1 varies from 101 to 150 cubic mils. Only tool A+ meets the criteria as evident from the plot. Part 2 varies from 150 to 200 cubic mils. In this range, only tool A+ is able to fully meet passing criteria. Tools B and C are barely failing criteria for BGA and QFP pads, respectively. Part 3 varies from 201 to 250 cubic mils. Tools A, A+, C, and E meet the passing criteria.

Similarly, Phase 3 solder paste volume range is broken down in two subparts, as shown in Figures 7a and b. Part 1 varies from 251 to 499 cubic mils. Most tools are able to meet the passing criteria except for tool D, which is barely failing the upper limit. Part 2 varies from 500 to 900 cubic mils. We observe almost all of the tools passing the criteria except tool C, which is barely trailing by a few percentage points on the upper limit of the passing criteria.

Page 2 of 3

• < Previous Page
• Next Page >