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Design Requirement of SMT PCBs Part Two: Settings of Pad-Trace Connection, Thru-Holes, Test Point, Solder Mask and Silkscreen

Settings of Pad-Trace Connection

• When pad is connected with the ground with a large area, cross grounding and 45° grounding should be first considered.

• The length of leads drawn from the ground with a large area or power lines should be more than 0.5mm and the width less than 0.4mm.

• The traces connected with rectangular pads should be drawn from the center of the long side of pad with angle generation avoided.

Design of connection between rectangular pads and traces | PCBCart

• Design of leads between IC circuit component pads and leads drawn from pad is displayed in Figure 2.

Design of leads between IC circuit component pads and leads | PCBCart

Settings of Thru-Holes

• Settings of Thru-holes (Through holes) in the application of reflow soldering:

a. Generally, the diameter of Thru-holes is at least 0.75mm.

b. Thru-holes can't be placed under other components except components such as SOIC and PLCC.

c. Thru-holes mustn't be directly placed on the pad or its expanded part and corner, which is shown in Figure 3.

d. There should be a fine line with soldermask film between Thru-hole and pad. The length of the fine line should be 0.5mm+ and the width 0.4mm+.

Thru-holes in the application of reflow soldering | PCBCart

• Thru-holes should be placed in the pad or near the pad in the process of wave soldering, which is helpful for gas release.

Test Point

• Some test points can be placed on PCB, either holes or pads.

• Setting requirement of test points is the same as that of reflow soldering through holes.

• Probe test supports through holes and test points.

In the application of online test, several probe tests should be placed on PCB to support Thru-holes and test points and when they are connected, lines should be drawn from any place of routing. The following aspects should be carefully considered:

a. The minimum spacing must be considered when probes with different diameters are for ATE (automatic test equipment).

b. Thru-holes mustn't be placed on the expansion part of pad, similar with the requirement of reflow soldering through holes.

c. Test points mustn't be determined on soldering points of components.


• PCB should pick up the surface finish of HASL (hot air solder leveling).

• Size of soldermask pattern should be bigger than pad surrounding edge by 0.05-0.254mm in order to stop soldermask paste polluting pad.

• In the cases of narrow spacing or no leads passing between pads, the soldermask pattern in Figure 4(a) can be followed; in the cases of leads passing between pads, the soldermask pattern in Figure 4(b) can be followed in order to avoid soldering flux bridge connection.

Soldermask pattern | PCBCart

Silkscreen Pattern

Generally speaking, the silkscreen pattern of components should be marked in the silkscreen layer, including silkscreen symbol, component tag number, polarity and I pin symbol of IC. Simplified symbol can be utilized in the case of high-density and narrow spacing, which is displayed in Figure 5. For special circumstances, component tag number can be omitted.

Silkscreen pattern | PCBCart

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Helpful Resources
Elements Ensuring Excellent PCB Pad Design for QFN
Solder Mask and Its Design Tips
Design Requirement of SMT PCBs Part One: Bonding Pad Design of Some Ordinary Components
Design Requirement of SMT PCBs Part Three: Component Layout Design
Design Requirement of SMT PCBs Part Four: Mark
Full Feature PCB Manufacturing Service from PCBCart - Multiple Value-added options
Advanced PCB Assembly Service from PCBCart - Start from 1 piece

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