Setting of Print Parameters for Fine-Pitch Printing

1. Introduction

Surface mount technology (SMT) is one of the mounting methods for printed circuit boards and solder paste is widely used as joint material between the PCB and component. As described in Fig. 1, a schematic image of SMT process, first solder paste is printed on the PCB using a stencil. Then In the next stage, the components are mounted and finally the solder paste is melted in a reflow oven to form the solder joint. Additionally, there are equipments like SPI are installed in between printer-mounter to check the quality of print.

Fig. 1 Schematic image of surface Mounting Process

Nowadays, demands for small and light portable devices, such as a smart phone, are increasing. Thus, electronic devices are expected to be compact, light-weight and high-functioning. To meet these requirements, it is necessary to mount many components in a small area. This trend helped the advancement in the reduction of component size (see Fig. 2, Table 1) and high-density mounting technology. As a result, printing technology which is compatible with fine-pitch stencil aperture is essential.

Fig. 2 Transition of Chip Component Size

Table 1 Size of Component

IPC stipulates solder powder size under section IPC J-STD-005A, from large size power Type 1 to small size powder Type 7. Currently, Type 4 solder powder is the commonly used solder powder; however, to comply with the fine-pitch print design, smaller solder powder such as Type 5 or Type 6 are being focused. Table 2 summarizes KOKI’s solder powders for fine-pitch design and Fig. 3 shows their grain size distribution. This article discusses setting of print parameters for optimal fine-pitch printability with smaller (type 5 and 6) solder powders based on in-house printing tests.

Table 2 Solder powder sizes of KOKI

Fig. 3 Solder Powder Size and Grain Distribution

2. Printing Test Method

Using metal stencil and metal squeegee, consecutively 10 test PCB’s has been printed for each solder pow der type. PCB’s containing the components mentioned in Table 3 and their aperture size is indicated in Table 3. Stencil has been designed according to the components required aperture size in various stencil thickness of 50µm and 80 µm. Additionally, solder paste transfer rate can be estimated according to the solder power size, stencil thickness and print pressure in relation to the aperture size. Table 4 shows the summary of test condition used for printing. For this test, SAC305 (Sn3.0Ag0.5Cu) alloy has been used with same type of flux regardless of the grain size to clarify the impact of the grain size.

Table 3 components and aperture size

Table 4 Print conditions for test

3.1 Results and Discussion - Solder Powder Grain Size

If the grain size is too big for the selected stencil aperture size, there may not be enough solder paste deposited and mostly the aperture may be clogged. In general, aperture diameter is recommended to be designed 8 times larger than the average grain size of the solder powder. Table 5 shows the minimum recommended aperture size requirements for each grain size of the solder powder. Based on the required aperture size and component size, stable printability may not be obtained for 0402 chip with Type 4, 03015 chip with Type 5 and 0201 chip with Type 6.

Table 5 Solder Powder Size and Stencil Aperture

Average print transfer rate is plotted in Fig. 4 for the 10 test PCB’s with different grain size. As expected, print transfer rate is reduced noticeably for type 4 solder powder, as the size of the aperture size becomes smaller from 0602 to 0201. Although Type 5 and Type 6 did not show any significant drop in the print transfer rate on any given stencil aperture size whereas Type 4 was not sufficiently printed on apertures smaller than 03015 chip, due to its grain size. However , Type 5 has performed better transfer rate than type 6 despite having large grain size than type 6. The reason behind the fall-out of transfer rate of type 6 is due to the surface area, which increases as the powder size decreases. This enhances the adhesion between solder powder and stencil aperture wall and leaves more solder paste on the stencil aperture itself. Though Type 5 has performed with highest print transfer rate, as indicated in Fig. 5, Type 5 displays wide print fluctuation, which can be as low as 0% when small aperture such as 0201 is used. Type 6 can be a good choice as the print to no print fluctuation is less compared to type 5. On contrary, Type 6 printed PCB’s do have certain volume of solder paste across all tested aperture sizes (see Fig. 6 and Table 6). When the stencil size is small but the grain size is large, less solder powders can be deposited in the aperture which reduces adhesion between the lands on PCB and solder paste, causes the solder paste to remain on the stencil upon the stencil release. This may have caused such no print/low print variation in Type 5 solder powder print transfer rate.

Fig. 4 Print Transfer Rate by Solder Grain Size

Fig. 5 Range of Print Transfer Rate of Type 5 Solder Paste

Fig. 6 Range of Print Transfer Rate of Type 6 Solder Paste

Table 6 Print Transfer Condition

3.2 Results and Discussion - Stencil Thickness

It is recommended that optimum stencil surface area ratio (A/R) between stencil thickness and aperture size is needed for optimal printability. When the stencil is designed the aspect ratio is expected to maintain 0.6 or larger for square and circular aperture openings to achieve optimum printability, which can be estimated by the following formula below;

A/R= (Stencil Aperture Area) / (Stencil Aperture Side Wall Area) ≧ 0.6

For circular and square apertures, simplified equation,

A/R = D / 4t ≧ 0.6 D=diameter or length of circular/square aperture t=thickness of the stencil

Table 7 summarizes the A/R for stencil aperture by chip size. When using 50 µm thick stencil, 0201 chip or smaller may display inconsistent print results. Similarly, if the stencil is 80 µm thick, 03015 chip or smaller may display inconsistent print results as per the calculation.

Table 7 Aspect Ratio according to the land size

Fig. 7 is a chart plotting the average print transfer rate of 10 printed samples for each stencil thickness. The aperture of 03015 chip size or larger, print transfer rate is higher on 80 µm thick stencil. On the other hand, smaller apertures such as 0201 chip, 50 µm thick stencil can perform better print transfer rate than 80 µm. As seen in Table 7, A/R for the 0201 chip aperture on 80 µm thick stencil is 0.31, which is significantly lower than the ideal A/R of 0.6. Under such condition, not enough solder paste can be deposited and also the adhesion between the PCB and solder paste can be reduced significantly. In addition, the stencil aperture area is larger for thicker stencils like 80 µm, excess amount of solder paste will be lifted along with the stencil during stencil release.

Fig. 7 Stencil Thickness and Print Transfer Rate

3.3 Results and Discussion - Print Pressure

Fig. 8 is a chart plotting the average print transfer rate of 10 printed samples for each print pressure with stencil thickness of 50 µm as listed in Table 4. If the print pressure is too strong, squeegee scrapes off excessive solder paste. This may explain the low print transfer rate for higher print pressure. Therefore, to obtain good printability at fine-pitch lands, print pressure is recommended to be maintained low but not too low, as there would be a smear on stencil.

Fig. 8 Print Pressure and Print Transfer Rate

4. Conclusion

Printability test with various grain size of solder paste, stencil thickness and print pressure has been studied. Based on the analysis of the data, following conditions can be implemented to improve the printability at the fine-pitch design.

  1. Type 5 solder powder performs stable printability. However, smaller lands and aperture of small components, i.e. 0201 or even smaller chip, Type 6 is more adequate due to the advantage of small grain size.
  2. To obtain optimal print results at smaller lands such as 0201 chip, use a solder paste with small solder powder size and a thin stencil.
  3. Higher the print pressure lower print transfer rate can be achieved. It is preferable to use lower print pressure but care must be taken to avoid smearing on the stencil, otherwise some other print defects may occur.

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