There are 2 general methods of placing components onto a PCB: THT and SMT. Combining the two on a board gives us the choice of using Mixed Technology.
Incorporating the technologies into your board requires different types of components. These components are differentiated by their packages - that is, how they are represented. DIP, BGA, QFP, and QFN are several package types to choose from, depending on your choice of placement technology.
THT or Through-Hole Technology mounts PCB components by inserting their lead throughout the PCB's holes and soldering it on the other end of the PCB. The components used are unique, they all have 2 or more pins attached to their body, which are longer than the PCB's thickness (typically 1.2 mm).
The Through-Hole Technology implements one main package: The DIP (Dual In-line Package).
The package refers to components which are housed with either molded plastic or ceramic, with long parallel leg-like leads emerging from its sides. These leg-like leads, or pins, are inserted into the drilled holes on a PCB to be mounted.
In simpler components such as resistors and inductors, the package has only two leads. These leads are arranged in either an axial or a radial manner.
In axial packages, the lead emerges from two alternate sides of the component. In radial packages, the lead emerges from the same side of the component.
In order to bond these THT components into the board, wave soldering method is used. Wave or flow soldering is the process of securing THT components onto the board by passing a preheated PCB with THT inserts through a pool of solder. The solder will cool down and create a bond between the leads of the THT component and the bottom pad of the PCB.
When dealing with small batches or more complex designs, hand soldering is often preferred for faster and more detailed soldering.
SMT or Surface Mount Technology is a modern circuit board assembly technology, which encompasses the miniaturization of components to achieve high reliability, high density, low-cost circuit boards. It enables batch production with a systematic automation process.
The Surface Mount Technology is further developed into several categories:
One of the most common technologies implemented with SMT is the SOIC, or Small Outline Integrated Circuit. SOIC is generally used for devices with simple functions or discrete devices, such as transistors, logic devices, and small ICs. The technology typically has a few pins which are distributed on the components' two sides. Other than the SMT's reflow solder process, securing SOICs are also done by applying red glue into the components.
SMT devices can be attached to the PCBs in a fast automated method with the reflow soldering process. Reflow soldering is using solder paste to stick SMT components onto the PCB pads, before having the whole setup heated up to create a permanent bond.
The Mixed Technology incorporates both of THT and SMT in one board, aiming to take the specialties of both.
Incorporating both technologies gives the product a mixed advantage of both SMT and THT services. The embedded THT technique gives the PCB a more robust property than regular SMTs. On the other hand, SMT supports more components within the small area, giving a denser working area than regular THTs. The choice of components used are also wider.
However, implementing 2 technologies require multiple types of soldering. With small batches of PCB, hand soldering can be used. However, when higher volume of PCBs are required, the board will need more complex soldering processes. Hence, the production of Mixed PCBs won't be as fast and as precise as regular SMT PCB will be, and the cost of the production is usually higher.
These Mixed PCBs are commonly used in LED lighting products, server boards, video processing applications, communication hardware, sensor boards, IoT hardware, industrial controller assemblies, smartphone accessories, or CPUs.
|Pros||✔️ More mechanical stability and robust|
✔️ Easier for small-batch production
✔️ Components are easier to be removed or resoldered
|✔️ Compact configuration|
✔️ Cheaper manufacturing costs for batch production
✔️ Faster production process
✔️ Precise soldering position
|✔️ Better mechanical stability|
✔️ More compact configuration
✔️ More component choice
|Cons||❌ Mechanical holes must be established by drilling, which is expensive and time-consuming|
❌ Limit routing areas in multilayer board
❌ Components take up more space
❌ Soldering is less reliable, non-repeatable, costly and has lower efficiency
|❌ Poor connection strength between components and board|
❌ Not suitable for high-temperature environments
|❌ More time-consuming and complex production than SMT|
❌ Higher manufacturing cost
A pad is a small surface of copper in a printed circuit board that allows soldering the component to the board. You can think of a pad as a piece of copper where the pins of the component are mechanically supported and soldered.
There are 3 types of commonly used pads based on the technology package used:
Through-hole pads are intended for introducing the pins of the components, so they can be soldered from the opposite side from which the component was inserted. These types of pads are very similar to a through-hole via. These pads can be either plated or non-plated.
The structure of through-hole pads can be called the pad stack, which consists of:
These are used for soldering the SMT components such as capacitors and inductors on the same surface as it was placed.
Surface mount pads are simpler than through-hole pads, consisting of:
Solder paste stenciling is a process of using a uniquely designed stencil to quickly apply solder paste across your PCB. These solder paste areas indicate the location of the components or pads (especially for SMT components) to be soldered onto the board.
The solder paste used is made up of a mixture of solder powder and flux. The solder powder is mainly composed of a mixture of tin-lead, tin-bismuth, or tin-silver-copper alloy.
Solder paste serves several functions, which includes:
The procedure of designing solder stencils are as follows,
The process of solder stenciling is done by either chemical corrosion, laser cutting, or a combination of laser cutting with electropolishing.