Solder Paste Printing Guide for SMT Components (3)

4 Print Quality Inspection

The currently used methods for the detection of stencil printing quality include visual inspection (using a microscope equipped with a light source), two-dimensional inspection, three-dimensional inspection equipment, X-ray inspection equipment, and automatic optical inspection equipment with a vision system. When testing solder paste printing quality, different inspection tools and methods can be used depending on the type of component.

4.1 Comparison of Manual Inspection and Automatic Inspection

The visual inspection method is used to inspect the printed components for pass and fail. Generally, the operator uses a ring illuminator or a microscope to inspect the sample board (in some cases, the whole board is inspected), and only when necessary, Correct the operation. This is one method of monitoring the lowest cost of the process, but the result of feedback is that the pass rate is very low. Visual inspection after printing can reduce the amount of rework by applying corrective operations in the most cost-effective printing process. However, visual inspection is based only on subjective awareness. Tests on visual inspection products show that only 80% are reliable. With the help of a visual inspection of a benchtop instrument, a benchtop vision detector. Using this instrument to measure the height of the solder paste, the amount of paste applied was calculated and the reproducibility was improved, but the artificial non-uniformity could not be completely ruled out. At present, there are still many manufacturers using visual inspection methods for testing.

The main difference between visual inspection, off-line inspection, and automatic on-line inspection is that automatic on-line inspection equipment detects more defects than the first two devices before the board is unloaded from the printing equipment. Automatic detection equipment can implement up to 100% of various levels of detection. This device not only monitors the process but also collects real data needed for process control.

Automatic on-line 3D solder paste inspection equipment offers different levels of speed, performance, and price, but only measures solder paste height, solder paste volume, and area on the board. Several stencil printer manufacturers have introduced built-in 2D area inspections, and some manufacturers have introduced field inspection equipment to test solder height and solder volume.

4.2 3D detection

The amount of solder paste is the best parameter to reflect the quality of the board. The focus of competition is also on maintaining the consistency of solder paste volume. (An assembly contractor reported that when testing a 2000 batch of 380-pin BGA-loaded BGA printed boards, after checking the board's solder paste, no board defects were found in the final test. ) Refer to the details of the test results provided in Table 6 for inspection.
Table 6

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Possible causes and countermeasures

Large size and high height
1) Contaminants on the circuit board lift up the template a) Clean the surface of the circuit board b) Clean the bottom of the template
2) The template is embossed a) Replace the template b) Adhere the damaged surface with tape and manually weld the affected area
3) Solder balls remain on the surface of the PCB during the HASL process
4) Excessive epoxy due to clogging of through-holes a) Removal of excess solder paste b) Return of printed board to supplier

Small size and height
1) Scraper is too fast
2) The template is released too fast, causing partial solder paste to remain on the template

Right size, high height
1) Excessive diffusion of solder paste a) Decrease the lifting speed b) Delay the falling speed c) Reduce the ejection speed
2) Too many open burrs a) Compare open and round ports to evaluate template design

Small size and height
1) Partial solder paste collapse at the end of the opening
2) The solder ball on the template is too small

Small size and low height
1) Partial release of solder paste in openings

2) Solder Paste Drying a) Replace solder paste b) Air is too dry, increase humidity

3) excessive collapse a) scraper speed too fast b) temperature of the solder paste is too high c) too much moisture absorbed by the solder paste
4) No solder paste in the template
a) Fill in solder paste

Large size and height

1) The bottom of the template is dirty
a) Cleaning template
2) Open side solder flow
a) Scraper pressure, adjust pressure
3) Cracks in the opening of the die plate between the pads
a) Change the template

The purpose of process monitoring is to reveal potential unsatisfactory solder paste parameters that are insufficient for solder to be determined before solder bridging results in bridging or until final inspection. The sooner the defect is found, the higher the cost of the repair. If defects are found before reflow, there is no detrimental effect on solder joint reliability. Therefore, off-line inspection or automatic on-line monitoring eliminates the problem of returning printed boards back to the production line, which saves money and increases reliability.

As a contract manufacturer points out, assembly companies expect to increase profits. The defects detected in the circuit test (ICT) must go through 15 steps, including work records, rework and assembly board feedback to the production line before retesting. In general, the cost estimates for defects or waste products generated at each stage of the printing process typically use a fast calculation equation: printing defects, $0.50 after reflow; ICT $5.00 and field failure $350.00. The detection of defects in the first phase resulted in a natural increase in the savings, which clearly demonstrated the cost of the process control equipment.

4.3 X-ray inspection

For a long time, X-rays have been used as an effective tool for visual inspection. This tool can be used for start-up, sampling and failure analysis of printing processes. A lot of work has been done in preparation for the X-ray machine to automatically produce the ground. However, some shortcomings of this type of X-ray machine have reduced the speed of its batch acceptance. For example, a 3D system can detect defects that are not detected by the 2D system, and it is even more incredible that it is vice versa. The filler on the bottom of the BGA is often obscured by the shadows of the surrounding filler bumps or cylinders (typically 90% lead wires), which presents great difficulties for inspection. The solder bridging on the QFP leads often leaves the board and extends to the leads, making it invisible to the 3D inspection instrument and causing missed inspections. The perception of holes is inconsistent; some say that holes are needed for ductility, while others believe that holes will reduce the performance of solder joints.

In addition to three mature production line technologies (electronics, software and servo motors), X-ray also applies two necessary additional technologies - high voltage and ionizing radiation, and begins assembly with BGA. This is a reliable process condition. , And then screen printing solder paste. However, the conditions for installing an automatic X-ray detector on the most mature assembly line are very clear. The evaluation of this technology is generally high, but its strength and simplicity are low.

in conclusion:

One of the most important design concepts in template design (opening size and template thickness) is the aperture/area ratio. As the size of electronic packaging products becomes smaller and smaller, the center-to-center spacing of I/O leads also shrinks. This means that the I/O board pad opening is also smaller. Due to the miniaturization of the template opening, the aperture/area ratio is also reduced. Therefore, more stringent requirements are imposed on the performance of the template. When selecting the stencil technology, it is the basic requirement to first consider the aperture/area ratio and maintain the aperture/area ratio above 1.5 and 0.66, respectively.

Overprinting (when the template opening is larger than the board pad) is useful for several applications, including intrusive reflow, fine pitch BGA, and ceramic BGA.

Author: Fan Yaonan