Distance requirements for solder paste process SMD pads and inline pads?

In the solder paste process (SMT), the minimum safe distance between the SMD pads and the inline pads is recommended to be 0.4mm to 1mm. this distance effectively prevents solder paste bridging, component interference, and ensures soldering reliability. For specific applications, refer to IPC-7351 standard and adjust with actual manufacturing capability.

Detailed requirements and basis

1. Safety distance norms

Basic requirements:

The distance between adjacent pad edges should be greater than 0.4mm to prevent tinning or short-circuiting during soldering.

The distance between direct insertion pads and SMD pads is recommended to be at least 1.5mm (mixed assembly scenario) to avoid component interference and soldering defects.

Special requirements for solder paste process:

For pad spacing ≤ 0.5mm, it is necessary to ensure that the solder paste covers 100% of the pads with no offset or bridging.

When the pad spacing is 0.65mm~1.25mm, the solder paste offset is allowed to be no more than 10%~15%, but it needs to be free of defects after furnace.

2. IPC standard references

IPC-7351 standard:

Pad design needs to consider component type, thermal management and electrical characteristics to ensure solder reliability and manufacturability.

Three pad pattern geometries are provided (Density Class N (High Density), M (Medium Density), L (Low Density)) for different assembly density requirements:

Density class N: For high reliability requirements with large pad extensions for easy rework, such as portable devices or automotive electronics.

Density class M: Suitable for medium-density assembly scenarios with moderate pad protrusion, such as QFP packages.

Density class L: Applicable to high-density assembly, pad minimisation to save space, such as 0402, 0201 device packages.

3. Practical Design Recommendations

Minimum safety distance:

Recommended value: 0.5mm~1mm, depending on component size and manufacturing process accuracy.

High-density layout:

Use oval or oblong pads to reduce the risk of continuous soldering.

For pad spacing <0.4mm, spread white oil or adjust the pad shape (e.g. diamond, plum blossom).

Manufacturing process adaptation:

When the pin aperture of direct inserted components exceeds 1.2mm or the pad diameter exceeds 3.0mm, it is recommended to design diamond or plum shaped pads.

Single-panel pad diameter or minimum width of 1.6mm, double-panel weak line pad aperture plus 0.5mm can be.

4. Frequently Asked Questions and Solutions

Solder paste bridging:

Ensure pad spacing ≥ 0.5mm, or use soldermask (spread white oil) to isolate.

Control solder paste printing thickness (stencil thickness ±0.02mm~0.04mm).

Component offset:

Mounter precision needs to meet ≤ ± 0.05mm, pad design needs to be reserved for alignment margin.

Thermal management:

High-power component pads need to increase the heat dissipation holes or expand the copper foil area.

Final Recommendation

Design priority to follow the IPC-7351 standard, and combined with the PCB manufacturer's actual processing capabilities (such as minimum line width, aperture accuracy) to adjust the pad spacing to ensure production yields. Key parameters include:

Mixed layout with direct insertion and SMD pad spacing ≥ 1.5mm;

Solder paste process pad spacing 0.5mm~1mm;

High-density layout pad spacing ≥ 0.4mm (with special shapes or soldermask).