PCBA Tech - Introduction of SMT patch template design guidelines

SMT patch template design guide

Stencil, also known as SMT stencil, SMT screen, and SMT steel mesh, is used to quantify solder paste or patch glue, and is a key tool to ensure the quality of printed solder paste/ patch red glue.

The thickness of the template, the size of the opening, the shape of the opening, the state of the inner wall of the opening, etc. determine the amount of solder paste printed, so the quality of the template directly affects the amount of solder paste printed. With the development of SMT to high-density and ultra-high-density assembly, template design becomes more important.

Template design is one of the important contents of SMT manufacturability design

Template design content

Template thickness

Template opening design

Selection of template processing method

Step/release template design

Hybrid technology: through-hole/surface-mount template design

No-wash opening design

Template design of PBGA

Template design of ceramic ball grid array (CBGA)

Template design of micro BGA/chip-level packaging (CSP)

Hybrid technology: surface mount/flip chip template design

Glue template opening design

Craftsman MT stainless steel laser template production outsourcing procedures and process requirements

1. Template thickness design

Stencil printing is contact printing, and the thickness of the stencil is a key parameter that determines the amount of solder paste.

The thickness of the template should be determined according to the assembly density of the printed board, the size of the components, and the spacing between the pins (or solder balls).

Generally, steel sheets with a thickness of 0.1mm to 0.3mm are used. For high-density assembly, the thickness of 0.1mm or less can be selected.

Generally, there are both general-pitch components above 1.27mm and narrow-pitch components on the same PCB circuit board. Components with a pitch above 1.27mm need to be 0.2mm thick, and components with a narrow pitch need to be 0.15-0.1mm thick. In this case, the thickness of the stainless steel plate can be determined according to the conditions of most components on the PCB, and then the amount of solder paste leakage can be adjusted by expanding or shrinking the opening size of the individual component pads.

When the disparity in the amount of solder paste is relatively large, the template at the narrow-pitch components can be locally thinned.

2. Template opening design

The template opening design contains two contents: opening size and opening shape

The size of the opening and the shape of the opening will affect the filling and release of the solder paste (demolition), and ultimately affect the amount of solder paste leakage.

The template opening is designed according to the printed circuit board land pattern, and sometimes needs to be appropriately modified (enlarge, reduce or modify the shape), because the structure, shape, and size of the pins of different components require different amounts of solder paste.

The larger the size disparity of the same PCB components and the higher the assembly density, the greater the difficulty of template design.

⑴ The most basic requirements of template opening design

Aspect ratio = opening width (W) / template thickness (T)

Volume ratio = opening area / hole wall area

The width to thickness ratio/area ratio of the rectangular opening:

Aspect ratio: W/T＞1.5

Area ratio: L*W/2(L+W)*T＞0.66

reserch shows:

Volume ratio> 0.66, solder paste release volume percentage> 80%

The volume ratio is less than 0.5, and the volume percentage of solder paste released is less than 60%

Three factors that affect the ability of solder paste to remove the film

Area ratio/width-to-thickness ratio, the geometry of the side wall of the opening, and the finish of the hole wall

The size [width (W) and length (L)] and template thickness (T) determine the volume of solder paste

Ideally, after the solder paste is released from the hole wall (demolition), a complete tin brick is formed on the PCB pad