Do you know what SMD (Solder Mask Defined) and NSMD (Non-Solder Mask Defined) are? What is the difference between SMD and NSMD? What are the advantages and disadvantages of SMD and NSMD? When should they be used?
Many people may say that they are in the PCBA electronics industry and have heard of SMD (Surface Mount Device) electronic parts, but don’t know what NSMD is. In fact, the SMD and NSMD mentioned here refer to the exposed copper foil pads or pads seen on the circuit board (pad layout design). This small detail in the PCB pad design that no one cared about before has become more and more important as electronic parts are getting smaller and smaller and solder joints are getting smaller.
So what exactly are SMD (Solder Mask Defined) and NSMD (Non-Solder Mask Defined) pad designs?
Today's PCB pads and traces are basically made of copper foil, but when we design PCBs, we don't expose all the copper foil, but only expose the pads that need to be contacted or soldered to avoid possible moisture short circuits or other problems in future use. At this time, we generally use the so-called "solder mask" to cover the copper foil that does not need to be exposed, so the position accuracy and ability of solder mask printing become very important for small pads.
What is SMD (Solder Mask Defined)?
SMD (Solder-Mask Defined Pad Design) is the name of a solder-mask (green paint/green oil) covering a larger area of copper foil, and then exposing the copper foil at the opening of the green paint (where the green paint is not covered) to form a pad. Because the size of this pad depends on the size of the green oil opening, it is called a solder-mask defined pad.
What is NSMD (Non-Solder Mask Defined), non-solder mask defined pad, copper foil independent pad
The NSMD (Non-Solder Mask Defined Pad Design) also known as the Copper Defined Pad Design is a design in which the copper foil is designed to be smaller than the opening of the solder mask green oil, which is a bit like an island in a lake. The size of the pad designed in this way basically depends on the size of the copper foil, so it is called an independent copper foil pad, also known as a non-solder mask defined pad.
What are the advantages and disadvantages of SMD (Solder Mask Defined) and NSMD (Non-Solder Mask Defined) pad designs? Is there any saying that which pad design is better? Is it more likely to solve the problem of BGA solder ball cracking?
Well! Those who ask this question or have this question are probably those who have been asked by someone in the company or have encountered problems with component solder cracking or falling off, especially the problem of BGA solder ball cracking.
I can tell you clearly that there is no pad design that can completely solve the BGA solder ball cracking problem. If you want to completely solve the BGA solder ball cracking problem, you have to start with the mechanism design. Many RDs only want to rely on pad design or strengthen the solder strength to solve BGA solder cracking. They have tried many methods, but the results are very limited. In the end, they almost all rely on improving the stress influence of the mechanism to achieve improvement.
In fact, the two pad designs, SMD and NSMD, each have their own advantages and disadvantages. They are also more advantageous in terms of solder strength and the bonding strength between the pad and the PCB, so it is really impossible to say which pad design is better. Before making a comparison, let us assume that the area of the SMD and NSMD pad designs is the same, so that the comparison is meaningful.
Advantages of SMD:
The actual copper foil size of the SMD pad is larger than that of the NSMD, and the pad is also covered and pressed with solder mask, so the bonding strength between the pad and FR4 is relatively good. During maintenance or rework, the pad is less likely to fall off due to repeated heating.
Disadvantages of SMD:
The solder strength of the SMD pad is relatively poor. This is because its relative tinning area is smaller, and the SMD pad is surrounded by solder mask. These solder mask oils expand and contract when flowing through the high temperature of reflow, which will also affect the tinning effect at the junction of the solder and the green paint.
It is easy to break from the surface of the pad when affected by stress.
It will be more difficult to route during PCB Layout because the spacing between pads is smaller.
Advantages of NSMD:
Because the pads of NSMD are independent copper foils, in addition to the front of the copper foil, the vertical sides around the copper foil can also be tinned during soldering. Relatively speaking, the tinning area of NSMD is relatively large, so the soldering strength is relatively good.
The actual area of copper foil of NSMD is relatively small, and it is easier for layout engineers to route (trace) because the pad size is relatively small, and the trace can easily pass between the pads of BGA.
Disadvantages of NSMD:
The bonding strength between the pad and FR4 is poor because the actual copper foil area is small. The pad is more likely to fall off due to repeated heating during maintenance and rework.
When affected by stress, it is easy to pull up the entire pad.
Flux and tin beads are more likely to remain in the area not covered by green paint.
When should SMD (Solder Mask Defined) and NSMD (Non-Solder Mask Defined) pad designs be used?
The pad shape of SMD design is generally more complete because it is less affected by the trace routing. It is recommended that small chip parts and pads with fine pins should use SMD design, such as resistors, capacitors, inductors, diodes (TVS diodes) and other parts below 0402. It is strongly recommended that 0201 and 01005 must use SMD pad design.
NSMD is not recommended for such small parts because most pads will be designed with routing to connect the pads with other lines. This is similar to the example of the island in the lake that needs to be connected by a bridge so that electronic signals can be connected. Imagine that the width of a bridge occupies 1/4 to 1/5 of the circumference of the island in the lake. These bridges can be regarded as copper foil for routing. In this way, the shape of the island in the lake (pad) will be deformed. Moreover, most pads have one in and one out routing. At most, there may be one pad connected to 3 to 4 routings. Such pads almost become SMD. When designing, PCB wiring engineers will not think so much. The opening of the solder mask will also be opened larger. This will cause the pad size on the solder joints at both ends of the same part to become different. What will happen if the pads at both ends of the chip are different? The amount of solder will be different, and tombstone phenomenon is prone to occur.
The impact of SMD or NSMD on soldering for other chips above 0603 is not that significant. However, if you can try to make all the pads on the same part the same size, it will still help improve the soldering quality.
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