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When designing a PCB layout, various questions inevitably arise. Is this PCB best suited for a single-layer, double-layer, or multi-layer configuration? How should you protect against ESD/EMI? Will the printed circuit board assembly (PCBA) use a single-sided or double-sided process? Should the selection of electronic components be through-hole devices, surface mount devices, or a combination of both? This article will primarily focus on the PCB soldering production processes.
(As a disclaimer, Workingbear is not an RD or an EE guy, so this article won’t delve deeply into ESD/EMI design and solutions. However, based on my knowledge, ESD is often related to the length of paths, similar to water seeking the shortest route during a flood. EMI concerns electromagnetic interference, which may involve radiation issues. For specific ESD and EMI solutions, consulting experts in these fields is advisable.)
How many layers should the PCB have? Single-layer, double-layer, or multi-layer?
The number of circuit layers in a PCB. Think of it as the floors in a building analogy. More layers mean that the same surface area can accommodate more circuits or “residents.” Layers are interconnected using vias or “staircases” between them.
More PCB layers generally mean you can design more functions within the same space. However, the cost of production also increases, similar to constructing taller buildings.
Typically, more advanced boards use more layers. There is usually at least one layer designated as a ground plane and one for the power voltage.
Single-sided or double-sided board? Single-sided soldering or double-sided soldering?
This decision affects the production cost of assembling the circuit board. If possible, try to place all components on one side of the board to significantly reduce the production process. Keep in mind that even if the other side of the board has just one component, a separate assembly line may still be needed for soldering. If you can consolidate this component onto the same side as the others, you can save on the cost of an additional assembly line.
Considerations for PCB Assembly Process: All Through-Hole, All Surface Mount, or a Mix of Both?
If possible, it’s best to go all through-hole (wave soldering process) or all surface mount (SMT process) to avoid the need for an additional, separate assembly line. SMT and wave soldering are different soldering processes. Many EMS factories are using wave soldering less frequently these days. If you can avoid the wave soldering process, it’s recommended because the energy costs for heating the entire solder bath to the required temperature can be substantial.
Does the SMT factory support red glue process?
Not every EMS factory offers the red glue process. It’s advisable to inquire about this before designing your PCB to avoid complications during production.
Additionally, the requirements for pad size and spacing for wave soldering and reflow soldering processes may vary for the same electronic component. It’s recommended to consult the SMT factory to obtain relevant Design for Manufacturability (DFM) requirements.
What is the “red glue” process in SMT? When should it be used, and what are the limitations?
How do EMS or SMT contract manufacturers calculate the cost of PCBA (Printed Circuit Board Assembly) services?
Generally, EMS (Electronics Manufacturing Service) or SMT contract manufacturers calculate the cost of contract manufacturing based on labor hours. Some may calculate it based on a fixed percentage of the material cost from the Bill of Materials (BOM). Additionally, they include the cost of all necessary tooling.
If your PCB assembly involves both SMT and wave soldering processes, SMT’s hourly labor rate is typically higher than that of wave soldering. This is because SMT equipment is more expensive, and operators require higher skill levels, resulting in higher wages.
In principle, if you can reduce the number of processes, you can save on production costs. Once you open a production line, whether you use it for only 10% or half of its capacity, the basic costs for the entire line remain fixed, and they do not decrease because you are using it less.
Let’s take a look at the five mainstream PCB assembly processes used in the electronics manufacturing industry!
PCB assembly can generally be categorized into the following mainstream processes: Through-hole components only? Surface-mount components only? Or a combination of through-hole and surface-mount?
PCB Assembly Process 1: Single-Sided Board with Through-Hole Components and Wave Soldering
This process is typically used for single-sided boards. All electronic components are through-hole components, including resistors, capacitors, and inductors. Operators insert these components into one side of the board and then pass it through “wave soldering” to solder the components onto the PCB. The distinctive feature of this type of board is that all the components are on the same side of the PCB, so they can be soldered in one pass through the wave soldering machine.
In the early days, when Surface Mount Devices (SMDs) were not common, this assembly process was widely used. The downside is that it requires more PCB space, and as technology has evolved, many components have transitioned to SMD types, leaving fewer through-hole component models. Consequently, you rarely see this process used for such boards today.
Occasionally, you might still find some cheap power supplies, transformers, or mouse boards that use this traditional process. The reasons include the affordability of through-hole components or the need for components that can handle high current or voltage. Opting for SMD components would significantly increase the cost, and most of these boards still use inexpensive paper-based PCBs. There are also some specialized boards that require components capable of withstanding high currents or voltages, for which suitable SMD alternatives may not yet be available.
PCB Assembly Process 2: Single-Sided Board with SMT Components and Wave Soldering
Process: Printing Solder Paste on Single-Sided of board –> mount SMD Components on Single-Sided of board -> Reflow Soldering -> Through-Hole Component Placement on Single-Sided of board -> Wave Soldering
This assembly process is primarily used for single-sided PCBs. On one side of the circuit board, it accommodates both SMD (Surface Mount Device) components and through-hole components. The process begins by applying solder paste to the board through an SMT stencil, followed by the placement of SMD components. Afterward, a reflow soldering process secures the SMD components to the board. Subsequently, the board enters the through-hole component placement station, where through-hole components are inserted into the board, either manually or through automated equipment. Finally, the assembly goes through a wave soldering process to secure the through-hole components.
Due to the trend of shrinking PCB sizes and the increasing affordability and popularity of SMD components, most consumer PCB assembly processes have transitioned towards the “SMD + through-hole” approach. In such modern assembly processes, a majority of electronic components on the PCB use SMD packaging. Through-hole components are reserved for those that cannot be replaced by SMD components or when cost considerations favor the use of through-hole components.
Therefore, this approach represents one of the most cost-effective PCB assembly processes available today.
PCB Assembly Process 3: Double-Sided Board with Double-Sided SMT + Red Glue + Through-Hole Wave Soldering
Process: First Side Solder Paste Printing for SMD Components -> First Reflow Soldering -> Second Side Underfill for SMD Components -> Second Reflow Soldering (Oven temperature adjusted as per underfill requirements) -> First Side Through-Hole Component Placement -> Wave Soldering
This assembly process represents an earlier method for soldering components on double-sided PCBs. During this transitional period when electronic components were shifting from through-hole to SMD packaging, circuit boards often featured at least 10 or more traditional through-hole components on a single board. Moreover, smaller components such as resistors, capacitors, and inductors (small chips) were still larger than 0603 in size, with some even using 1206 components.
As a solution, this process involved applying red glue on the second side of the board to secure SMD components without solder paste and subsequently passing the board through a wave soldering process after through-hole component insertion. Options one and two were suitable for single-sided boards, and this approach significantly increased the utilization of both sides of the board, reducing material of PCB wastage. However, if the second side of the board, which was already printed with solder paste to secure SMD components, went through the wave soldering process with numerous through-hole components, the solidified solder paste could melt in the wave solder bath, causing SMD components on the second side to fall into the solder pool. To address this, the red glue process was introduced. Red glue typically consists of an irreversible epoxy resin material that secures SMD components, preventing them from re-melting during wave soldering.
Usage Constraints for Red Glue:
Not all SMD components can undergo wave soldering with red glue; this process is typically suitable for IC components with dual-row extended pins or components with terminals at both ends, like resistors, capacitors, and inductors (small chips).
QFP components with pins extending on four sides have been processed through wave soldering, but their pin spacing must be sufficiently large to avoid short-circuit issues.
Furthermore, 0603 is generally considered the lower size limit for red glue processes. Smaller sizes can lead to red glue interfering with the pad positions, causing solder voids since controlling the amount of red glue is not as straightforward as solder paste. Additionally, very close pad spacing during wave soldering can lead to open circuits.
PCB Assembly Process 4: Double-Sided Board with Double-Sided SMT + Selective Wave Soldering for Through-Hole Components
Process: SMT Solder Paste Printing for SMD Components on the First Side -> First Reflow Soldering -> SMT Solder Paste Printing for SMD Components on the Second Side -> Second Reflow Soldering -> Through-Hole Component Placement -> Selective Wave Soldering or the Use of a Wave Soldering Carrier Fixture
As technology advances, this double-sided SMT assembly process has become one of the most common designs. Nowadays, the vast majority of electronic components are available in SMD packages. Only a few components that require mechanical insertion and for reasons of functionality and reliability still necessitate through-hole components to ensure robust soldering.
This assembly process has largely abandoned the “red glue” procedure. For the few components that still need wave soldering, it’s possible to consider using “mask wave soldering” templates or carriers to cover SMD components that have already undergone SMT soldering. This leaves only the locations where wave soldering is required exposed.
Related Articles:
Conditions for Using Selective Mask Wave Soldering
Additionally, you might explore the introduction of “selective wave soldering” equipment that has gained popularity in recent years. It utilizes a small solder wave and nozzles, controlled automatically by computer-driven processes, to solder through-hole components from below.
Related Articles:
Pros and Cons of Selective Wave Soldering Machines
However, both of these through-hole component soldering processes require a “keepout area” in the design. Therefore, you must consult DFM (Design for Manufacturability) requirements during the design phase.
Apart from the two mentioned wave soldering processes, there are more cost-effective alternatives due to the relatively small number of through-hole components:
Hand soldering: I strongly discourage manual soldering due to challenges in quality control. Common issues with hand soldering include soldering errors and solder voids, especially for components with fine pitch pins. This method demands highly skilled workers, and long-term manual soldering is also detrimental to the solderer’s health.
Robotic or automated soldering equipment: Automated soldering can range from affordable to high-end. These machines typically use soldering irons and solder wire but replace manual labor. They can be set up using a simple XYZ-axis table, although achieving precision and fine-tuning parameters may take some time.
There’s another soldering process worth mentioning called Paste-in-Hole (PIH) reflow soldering. Although it involves through-hole components, by elevating the material’s heat tolerance, you can treat through-hole components like SMD components, attach them with solder paste, and run them through a reflow oven for direct soldering. PIH comes with its own set of requirements; for more details, refer to the article “How to Process Through-Hole Components/Traditional Plug-Ins in Reflow Soldering (Paste-in-Hole, PIH).”
PCB Assembly Process 5: Double-Sided Board with Double-Sided SMT Reflowing Soldering
Process: SMT Solder Paste Printing for SMD Components on the First Side -> First Reflow Soldering -> SMT Solder Paste Printing for SMD Components on the Second Side -> Second Reflow Soldering (2nd reflow)
Currently, this process is widely employed in the assembly of smartphones and some consumer electronic products. This is mainly because modern smartphones typically feature only one Type-C or Lightning connector, and perhaps a headphone jack at most. Furthermore, smartphones are personal devices, and users tend to handle them delicately, avoiding rough insertions and removals.
For these reasons, it is entirely feasible to use SMD connector models. Designing appropriate retention and anti-lift mechanisms ensures that there are no connector detachment issues during the warranty period or the device’s lifespan.
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