1. In recent years, as the performance demands for smart terminal devices like smartphones and tablets have escalated, the PCB manufacturing industry faces a growing need for miniaturization and thinning of electronic components.
2. This demand has intensified with the emergence of wearable devices, leading to an even greater focus on component size reduction.
3. As components shrink further, the challenges in the PCB production process will also increase.
4. Improving the one-time throughput has become the ultimate goal for SMT process engineers.
5. Generally speaking, over 60% of defects in the SMT industry are linked to solder paste printing, a critical phase in SMT production.
6. Addressing solder paste printing issues effectively resolves most process challenges within the entire SMT workflow.
7. Currently, British 01005 SMD devices and 0.4 pitch BGA/CSP are widely utilized in SMT production, with a limited use of metric 03015 SMD devices.
8. Meanwhile, metric 0201 SMD devices are still in the trial production stage, with expectations for gradual adoption in the coming years.
9. To grasp the challenges posed by miniaturized components on solder paste printing, it is essential to first understand the area ratio of stencil printing.
10. The area ratio of stencil printing is crucial; if it fails to meet requirements (such as if the stencil is excessively thick), specific issues may arise.
11. When solder paste is printed and demolded, paste from small components may cling to the walls of the steel mesh, resulting in insufficient amounts of solder paste on the pads.
If the stencil opening area ratio does not meet the requirements (the stencil is too thick), the following issue may arise.
For the solder paste printing of miniaturized pads, the smaller the pad and the stencil opening, the more challenging it is for the solder paste to release from the stencil hole wall. To address the solder paste printing of miniaturized pads, consider the following solutions:
1. The most straightforward solution is to reduce the thickness of the steel mesh and increase the area ratio of openings.
As illustrated below, using a thinner steel mesh improves soldering on the pads of small components. If the substrate does not contain large components, this approach is both simple and effective. However, for substrates with large components, the soldering may suffer due to insufficient solder. Thus, for high-mix substrates featuring large components, alternative solutions are necessary.
2. Employ new steel mesh technologies to lessen the requirements for the opening ratio of the steel mesh.
1) FG (Fine Grain) steel mesh
FG steel mesh incorporates niobium, which refines the grain, reducing overheating sensitivity and temper brittleness while enhancing strength. The laser-cut hole walls of FG steel sheets are cleaner and smoother than those of standard 304 steel sheets, facilitating better demolding. The opening area ratio for FG steel mesh can be lower than 0.65. Compared to 304 steel mesh with the same opening ratio, FG steel mesh can be slightly thicker, thus minimizing the risk of inadequate solder in large components.
2) Electroformed steel mesh
The manufacturing process for electroformed steel mesh involves applying photoresist material onto a conductive metal base plate, creating an electroforming template through mold shielding and UV exposure, then immersing the thin template in electroforming liquid. This method resembles electroplating, with the difference that the nickel sheet can be detached from the base plate to create a steel mesh.
Electroformed steel mesh boasts several characteristics: it contains no internal stress, has very smooth hole walls, and can be manufactured to any thickness (up to 0.2mm, controlled by electroforming duration). Its drawback is a higher cost. The following image compares laser steel mesh with electroformed steel mesh. The smooth hole walls of the electroformed version enhance demolding after printing, allowing for an opening ratio as low as 0.5.
3) Ladder steel mesh
The stepped steel mesh can be selectively thickened or thinned. The thicker sections print solder pads requiring more paste, achieved via electroforming, which is more expensive. Thinning is accomplished through chemical etching, benefiting the pads of miniaturized components and improving demolding. Cost-sensitive users are advised to consider chemical etching for a more economical option.
4) Nano coating (Nano Ultra Coating)
Applying a layer of nano-coating on the steel mesh surface enhances repulsion of solder paste from the hole walls, thus improving demolding effectiveness and ensuring consistent solder paste volume stability during printing. This results in higher print quality and fewer cleanings of the steel mesh. Currently, many domestic processes apply only a single layer of nano-coating, leading to diminished effects after multiple prints. In contrast, some international practices involve directly plating the steel mesh with nano-coatings for improved performance and durability, albeit at a higher cost.
3. Dual solder paste molding processes.
1) Printing/Printing
This method utilizes two printing machines: one with a standard stencil for fine-pitch small component pads, and the second with a 3D stencil or stepped stencil for large component pads. While effective, this approach necessitates two printers and incurs higher stencil costs, especially when a 3D stencil and comb-shaped scraper are employed, impacting production efficiency.
2) Printing/Spray tin
In this scenario, the first solder paste printer handles small component pads, while a second inkjet printer applies solder paste to large component pads. While this method yields good solder paste formation, it is costly and less efficient, especially depending on the quantity of large component pads.
Users can select from these solutions based on their specific circumstances. In terms of cost and production efficiency, reducing stencil thickness, utilizing stencils with lower aperture area ratios, and adopting stepped stencils are generally more suitable options. For users with low output but high quality requirements and who are less sensitive to costs, the printing/jet printing option may be more appropriate.
2. This demand has intensified with the emergence of wearable devices, leading to an even greater focus on component size reduction.
3. As components shrink further, the challenges in the PCB production process will also increase.
4. Improving the one-time throughput has become the ultimate goal for SMT process engineers.
5. Generally speaking, over 60% of defects in the SMT industry are linked to solder paste printing, a critical phase in SMT production.
6. Addressing solder paste printing issues effectively resolves most process challenges within the entire SMT workflow.
7. Currently, British 01005 SMD devices and 0.4 pitch BGA/CSP are widely utilized in SMT production, with a limited use of metric 03015 SMD devices.
8. Meanwhile, metric 0201 SMD devices are still in the trial production stage, with expectations for gradual adoption in the coming years.
9. To grasp the challenges posed by miniaturized components on solder paste printing, it is essential to first understand the area ratio of stencil printing.
10. The area ratio of stencil printing is crucial; if it fails to meet requirements (such as if the stencil is excessively thick), specific issues may arise.
11. When solder paste is printed and demolded, paste from small components may cling to the walls of the steel mesh, resulting in insufficient amounts of solder paste on the pads.
If the stencil opening area ratio does not meet the requirements (the stencil is too thick), the following issue may arise.
For the solder paste printing of miniaturized pads, the smaller the pad and the stencil opening, the more challenging it is for the solder paste to release from the stencil hole wall. To address the solder paste printing of miniaturized pads, consider the following solutions:
1. The most straightforward solution is to reduce the thickness of the steel mesh and increase the area ratio of openings.
As illustrated below, using a thinner steel mesh improves soldering on the pads of small components. If the substrate does not contain large components, this approach is both simple and effective. However, for substrates with large components, the soldering may suffer due to insufficient solder. Thus, for high-mix substrates featuring large components, alternative solutions are necessary.
2. Employ new steel mesh technologies to lessen the requirements for the opening ratio of the steel mesh.
1) FG (Fine Grain) steel mesh
FG steel mesh incorporates niobium, which refines the grain, reducing overheating sensitivity and temper brittleness while enhancing strength. The laser-cut hole walls of FG steel sheets are cleaner and smoother than those of standard 304 steel sheets, facilitating better demolding. The opening area ratio for FG steel mesh can be lower than 0.65. Compared to 304 steel mesh with the same opening ratio, FG steel mesh can be slightly thicker, thus minimizing the risk of inadequate solder in large components.
2) Electroformed steel mesh
The manufacturing process for electroformed steel mesh involves applying photoresist material onto a conductive metal base plate, creating an electroforming template through mold shielding and UV exposure, then immersing the thin template in electroforming liquid. This method resembles electroplating, with the difference that the nickel sheet can be detached from the base plate to create a steel mesh.
Electroformed steel mesh boasts several characteristics: it contains no internal stress, has very smooth hole walls, and can be manufactured to any thickness (up to 0.2mm, controlled by electroforming duration). Its drawback is a higher cost. The following image compares laser steel mesh with electroformed steel mesh. The smooth hole walls of the electroformed version enhance demolding after printing, allowing for an opening ratio as low as 0.5.
3) Ladder steel mesh
The stepped steel mesh can be selectively thickened or thinned. The thicker sections print solder pads requiring more paste, achieved via electroforming, which is more expensive. Thinning is accomplished through chemical etching, benefiting the pads of miniaturized components and improving demolding. Cost-sensitive users are advised to consider chemical etching for a more economical option.
4) Nano coating (Nano Ultra Coating)
Applying a layer of nano-coating on the steel mesh surface enhances repulsion of solder paste from the hole walls, thus improving demolding effectiveness and ensuring consistent solder paste volume stability during printing. This results in higher print quality and fewer cleanings of the steel mesh. Currently, many domestic processes apply only a single layer of nano-coating, leading to diminished effects after multiple prints. In contrast, some international practices involve directly plating the steel mesh with nano-coatings for improved performance and durability, albeit at a higher cost.
3. Dual solder paste molding processes.
1) Printing/Printing
This method utilizes two printing machines: one with a standard stencil for fine-pitch small component pads, and the second with a 3D stencil or stepped stencil for large component pads. While effective, this approach necessitates two printers and incurs higher stencil costs, especially when a 3D stencil and comb-shaped scraper are employed, impacting production efficiency.
2) Printing/Spray tin
In this scenario, the first solder paste printer handles small component pads, while a second inkjet printer applies solder paste to large component pads. While this method yields good solder paste formation, it is costly and less efficient, especially depending on the quantity of large component pads.
Users can select from these solutions based on their specific circumstances. In terms of cost and production efficiency, reducing stencil thickness, utilizing stencils with lower aperture area ratios, and adopting stepped stencils are generally more suitable options. For users with low output but high quality requirements and who are less sensitive to costs, the printing/jet printing option may be more appropriate.
