SMD Pads | Key to Successful PCB Assembly

Introduction

Surface Mount Device (SMD) Pads are a vital component of the PCB fabrication process. Manufacturers prefer SMD pads for their many advantages.

Marketwatch predicts that the global market for surface mount technology will grow at a compound annual growth rate of 7.46% over the period.

According to Global News wire, The SMT market might reach US$4.5 billion by 2027. SMD pads are appealing to manufacturers because of their efficiency and low cost.

As SMD pads are so crucial in the modern electronics industry, PCBMay offers a variety of high-quality pads, so you get more options. PCBMay eliminates the need for extra layers in the board design by putting pads on both sides of the PCB.

It allows for more freedom in component positioning. As a result, manufacturing costs are reduced, and production speeds are increased.

We have discovered that our clients have several queries about SMD pads. So, we have chosen to address them all in one place. To provide you with the most recent and in-depth understanding of SMD pads, we have compiled this FAQ guide.

Let’s go in right now.

What Are SMD Pads?

Surface Mount Device pads (SMD pads) are tiny areas of electrical conductors used to attach electrical parts to a PCB. Installed on the outside layer of the printed circuit board, these pads are commonly square or circular in form.

To prevent any solder from flowing out of the pad during the reflow process, solder mask holes covering SMD pads are often made slightly smaller than the diameter of the pad. Fine pitch components, like ball grid arrays (BGAs), can be put on SMD pads.

On the surface of the PCB, there is a copper pad for BGA pads. The size of the pad depends on the BGA package. Copper pads might be bigger or smaller than BGA pads. It depends on the PCB’s design specs.

Most of the time, the copper layer on an SMD BGA pad is the same size as on the BGA itself.

When an electronic part like a resistor or capacitor is put on the PCB, its leads are soldered to the SMD pads. This ensures a good connection between the component and the PCB.

SMD pads are also more reliable and last longer than through-hole parts. They are less likely to get broken by vibrations or shocks, which is important in situations where the device might be handled roughly or put in harsh conditions.

SMD pads are more efficient and cheaper than through-hole components and are smaller and more reliable. By putting pads on both sides of the PCB, companies may simplify the board design by reducing the need for extra layers of the PCB, and manufacturers can reduce the need for additional layers in the board design.

SMD pads

SMD pads

What Are the Design Rules for SMD Pads?

SMD pads must be designed following strict guidelines to provide reliable connections between electrical components and the PCB. Consider these design rules if you want to make better SMD pads, or check to see if your manufacturer follows these rules.

Some of the most important guidelines to follow while designing SMD pads are as follows.

1. Use PCB Standard Packaging Library.

To guarantee appropriate assembly and durability of surface mount devices (SMDs), pads should be at least 0.25 millimeters thick and no and over three times the part’s hole. Leave 0.4 millimeters between pads.

When there are a lot of wires, the oval, as well as the oval plate, is the best choice.

If you’re using a double-sided PCB with weak soldering on one side, keeping the pad sizes small; any larger than the hole diameter + 0.5mm might cause unwanted welding.

Diamond or plummet pads should be constructed for holes bigger than 1.2mm or pad diameters more than 3.0mm to achieve good soldering.

Making teardrop-shaped plug-in components from a single sheet of copper foil is a good way to avoid the typical problem of broken foil during welding. Cover both sides of this foil.

2. The PCB pad size calculator

PCB designers no longer hand build footprints, which caused differences among manufacturers. Nowadays, trustworthy solutions eliminate mistakes and give current information. Partnering with footprint firms for your CAD packages is one solution.

They have done this kind of work for a long time and are very good at it. They can save you a lot of time and money by doing the research and development work for you.

3. Knowledge of different kinds of PCB pads

Most PCB pads are either soldered surface-mount pads or soldered through-hole pads. Solder pads are typically square or rectangular in form, with exact proportions depending on the component being soldered to them.

SMT pads design rules

SMT pads design rules

NPTH and PLTH:

When it comes to the soldered through-hole pads, you can pick among plated through-hole pads or non-plated through-hole pads. Both of these are pads in the shape of circles, squares, or oblongs with a hole in the middle to receive the lead of the solderable electrical component.

This makes it possible to mount a component with leads by putting the leads through the hole and soldering them to the pads.

4. PCB pad size issues in PCB designing

If the surface mount pads on a PCB aren’t manufactured correctly, it can cause problems during the soldering and assembly process. Some of these issues include:

Floating components occur when surface mount parts move during solder reflow if the pads are too wide or not positioned correctly. This can result in solder bridging and inadequate component distance, making thermal cooling, repair, and PCB testing difficult.

Incomplete solder joints: If the pads are too small or too close together, there may not be enough solder filets, leading to bad connections or no solder joints at all.

Solder bridging: When solder flows from large surface mount pads to another pad or metal feature on a separate net, it creates a direct short. To prevent this, designers can use CAD tools to design the pad shape with the appropriate solder mask and paste features.

Tombstoning is when a small surface mount component becomes thermally imbalanced during solder reflow, causing it to tombstone due to uneven melting of the solder paste on its pads.

How Are SMD Pads Properly Cleaned and Inspected After Assembly?

SMD pads should be cleaned and checked carefully to ensure they are not broken or full of dust. When cleaning a PCB after assembly, it is important to use a cleaning solution or solvent that is non-corrosive to the PCB and its components.

Professionals often use ultrasonic cleaning to remove dirt and flux residues on the pads and in the area around them. Once the SMD pads have been cleaned, look for any flaws with a magnifying glass or microscope.

Damaged SMT pads

Damaged SMT pads

SMD pads are often checked using automated optical inspection (AOI) and X-ray inspection, particularly for high-volume manufacturing.

What Is the Maximum Current-Carrying Capacity of SMD Pads?

The maximum amount of current that SMD pads are capable of carrying is subject to a variety of factors, including the following:

  • The size and shape of the pad
  • The thickness and type of pad
  • Solder coverage and quality
  • The width and length of the trace
  • The temperature and conditions for cooling

At 25°C, a trace that is 10 mils wide and 1 oz/ft2 thick can carry up to 0.6 A with a 10°C temperature increase. Depending on the configuration, currents of up to 50 A are conceivable.

What Is the Shadow Effect in SMD Pads?

A typical issue that arises during reflow soldering is the shadow effect on SMD pads. The shadow effect keeps some hot air or IR rays from reaching the smaller parts. This means the smaller parts don’t heat up properly, making it hard to solder them.

This can lead to bad soldering of the smaller parts, such as cold solder joints, tombstoning, or parts that aren’t lined up right. To reduce the shadow effect on SMD pads, consider the following recommendations.

  1. To prevent overheating and solder bridging, don’t utilize a single large pad between two linked SMDs, but connect smaller wires between the two pads.
  2. Due to the potential for heat stress and component damage, through holes should not be drilled on or near SMD pads.
  3. Make the copper on the edge pads bigger for IC pads with many pins to help heat escape and reduce the chance of the pads falling apart.
  4. Short-circuit the component to the ground or between parts to avoid solder overflow and shorts.
  5. Use solder masks and screen printing oil to cover the other side of the through-hole to stop the solder from escaping and generating problems like shorts.

SMD pads shadow effect

SMD pads shadow effect

How to Calculate SMD Pad Sizes?

Many variables, including the kind of component and its mounting manner, play a role in determining the proper pad size for SMD components. For example, BGAs have different requirements for pad size than either leadless or leaded packages. So, there are different ways to figure out the size of the pad.

The formula for the size of the pad is:

Pad size = Lead size + 2 * Solder mask opening + 2 * Annular ring clearance

The Generic Requirements for Surface Mount Design and Land Pattern Standard gives guidelines for the land patterns of standard components. By this, you can figure out the sizes of the pads.

SMD pad sizes

SMD pad sizes

What Equipment and Tools Are Required to Apply and Rework SMD Pads?

To install and repair SMD pads, you will need certain specialized equipment and tools, including the following:

  • A fine-tipped soldering iron
  • Wire or paste for soldering
  • Flux
  • Tweezers
  • A microscope or a magnifying light
  • A hot air gun or a reflow oven

The most basic way to solder SMD pads is to put some flux on the PCB footprint and then a small amount of solder on one pad. A pair of tweezers will come in here as you carefully place the SMD component into its PCB footprint.

Lastly, while heating the pad with the soldering iron, carefully press the component to the pad. You can do the same procedure with the other pads.

Reworking SMD pads

Reworking SMD pads

Some packages, like BGA, iLCC, CSP, etc., have pads underneath them. You will need a hot air gun or a reflow oven to melt the solder paste on these packages. You can use either a stencil or a syringe to apply solder paste.

Then, you can heat the part on top of the paste until it flows back together. After the joints have cooled, you can look at them with a magnifying lamp or a microscope.

For packages like QFN, DFN, etc., with large thermal pads under them, you will need to use a technique called “drag soldering.” You can put flux and solder paste on the thermal pads and the pads around the edge.

Then you can put the component on top of them and use a hot air gun or a reflow oven to heat them up until the solder reflows. After the solder has cooled, you can use a flat-tipped soldering iron to pull any extra solder away from the thermal pad.

Can SMD Pad Sizes Be Larger Than PCB Footprints?

SMD pad sizes can be bigger than PCB footprints, but it’s not a good idea because it can cause problems when making and putting together the board.

Considering the dimensions of the SMD pads and other crucial components of the footprint is of utmost importance during the process of PCB footprint design.

PCB footprint

PCB footprint

Following the clearance requirements related to the solder mask opening layer is essential to avoid Design for Assembly (DFA) mistakes and flaws during assembly.

Working with your fabrication team and assembler can give you essential information about ensuring your board is free of flaws and meets the requirements.

What Is the Typical Thickness of SMD Pads?

SMD pad thickness is typically determined by the PCB design and fabrication method. SMD pads may have a thickness that varies anywhere from 0.008 inches (0.2 mm) to 0.02 inches (0.5 mm).

The IPC-7351B standard specifies minimum and maximum pad widths and thicknesses for specific SMD components. These proposals are based on the type of packaging and the size of the component.

SMD Pad vs. NSMD: Which One to Use?

Non-Solder Mask Defined (NSMD) pads are often recommended for use with surface mount components by EMS manufacturers. NSMD pads provide more exact registering of copper artwork compared to solder masking, which is the traditional method.

The use of NSMD pads can boost the precision of your PCB design and guarantee that your surface mount components are installed properly. The SMD pad definition may produce stress concentration spots that might break solder joints if fatigued.

In NSMD, the pad dimensions are often reduced by an extra fifteen percent in comparison to the size of the BGA diameter. This is done to guarantee that the solder connections are subjected to equal pressure throughout. The primary layer routing channel may also be widened by using this method.

Additional information is available below to aid in the selection of either NSMD or SMD for your designs:

  • Use the proper pad sizes when constructing BGA 0.4mm pitch PCBs to guarantee great yields in big numbers.
  • BGA ball sizes at 0.4mm pitch are smaller than those at 0.5mm pitch; hence, excessively lowering pad sizes may lead to the inadequate solderable surface, probable failures, or rejected PCBs.
  • Using 0.4mm and 0.3mm pitch BGAs with non-solder mask-defined (NSMD) pads might cause bridging between pads owing to inadequate soldermask webbing between pads.
  • When pad diameters decrease, and solder mask webbing decreases, pad detachment rises during reflow or field use.
  • To prevent shorts in exterior layer pitches of 0.4mm or smaller, only have traces between pads on inner layers and no trace between neighboring BGA pads.

SMD Pad vs NSMD

SMD Pad vs NSMD

What Are the Advantages of SMD Pads?

SMD provides various benefits over the traditional through-hole technique, including the following:

SMD pads benefit microelectronics as they enable more components to be placed closer together on the board. This results in smaller and lighter devices.

Setting up SMD production is faster than through-hole production as solder paste is used instead of drilled holes, saving time and labor.

SMD allows components to be placed on both edges of the circuit board, increasing the component density and the number of connections. Additionally, SMD packages are small, enabling more traces to fit on the same layer.

Surface tension helps align components and rectify small placement mistakes during reflow, creating strong and secure solder connections.

Furthermore, SMD reduces connection resistance and inductance, improving high-frequency performance and signal quality. The compactness of SMT also allows for shorter signal routes, reducing heat generation compared to through-hole parts.

SMD technology is cost-effective for boards and material handling, making it a preferred option for high-volume PCB manufacturing. Overall, SMD offers a controlled and efficient process for making microelectronics.

What Are the Limitations of SMD Pads?

Although SMD provides a number of benefits, the technology also has several drawbacks:

When it comes to the SMD component packages, it is often not feasible to use sockets to install damaged components. Though SMD cuts down on the quantity of solder used in solder connections, there could be greater issues with solder joints’ dependability as a result of this reduction. Joint failures may occur when voids form in the solder.

In addition, SMD connections may be susceptible to being broken by thermal cycles that occur during operation. It may be essential to hire highly trained or professional workers and invest in pricey equipment in order to undertake component-level fixes or build models manually.

Most SMD component packages cannot be put in sockets to install and replace faulty components easily. Using SMD reduces the amount of solder used for solder junctions, which can raise concerns about solder joint reliability.

When exposed to excessive heat, the solder used in SMD may melt, limiting its use in electrical circuits with high heat levels.

Conclusion

SMD pads are an essential component in modern PCB design and assembly. SMD pads, widely used in modern electronics manufacturing, are essential to surface mount technology (SMT).

In order to simplify the assembly of circuit boards, SMT has allowed electrical components to be attached directly to the board’s surface. The adoption of SMD pads has allowed for the fabrication of smaller and more complicated circuit boards, all while lowering production costs and making better use of available board space.

At PCBMay, we are dedicated to providing you with the highest-quality SMD Pads for your board to give the best performance. We do not only give the best quality product but also keep the prices the lowest.
If you are looking for an SMD pads manufacturer, then PCBMay would be a no-brainer. We want to collaborate with you for your next project and build the best of both worlds. Please visit our website to request a quote or contact us anytime.

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