Observation and Measurement of Through Holes and Lands on PWBs
Smartphones, tablets, wearable devices, and other similar devices have been designed to be smaller, thinner, and with higher functionality. Because of this, PCBs and components have also been designed to be smaller, denser, and with more layers. Consequently, it has become more difficult to observe fine parts (such as through holes, lands, and pads) and to measure and evaluate 3D shapes (such as surface irregularities) for research, development, and quality assurance of printed circuit boards (PCBs) and printed wired boards (PWBs). This section introduces the basic knowledge of PWBs and PCB mounting as well as examples of observation and measurement of the details of PWBs using our latest 4K digital microscope.
- Types, Structures, and Characteristics of PWBs
- PCB Mounting Methods
- Observation and Measurement Examples of Through Holes and Lands on PWBs
- A 4K Digital Microscope That Strongly Supports the Research, Development, and Quality Assurance of PWBs and PCBs
Types, Structures, and Characteristics of PWBs
In PCB manufacturing, quality after component mounting depends on PWBs. The types of PWBs and the structure, characteristics, and part names of each type are explained below.
Types of PWBs
The representative types, characteristics, and structures are explained below with figures.
Single-sided board (one layer board)
Only one side of the substrate is printed with copper foil. Having only one layer, it is called a single layer board. Non-plated through holes, into which component leads or electrodes are inserted, are drilled or punched on the substrate, the insides of these holes are insulated because they are not plated with copper. Lands or pads on the substrate surface are covered with copper foil to be contacts with the components to mount. Single-sided boards are often used for mass-produced electronics for consumer use because of low manufacturing costs.
- A
- Non-plated through hole
- B
- Substrate
- C
- Copper foil
Double-sided board (two-layer board)
Both sides of the substrate are printed with copper foil. This type is called a two layer board. The insides of the through holes used for mounting components are conductive because they are plated with copper. The manufacturing costs are higher than those of single-sided boards. Yet, this type is widely used for electronics because the area for wiring and mounting is twice as large as single-sided boards, reducing the size of the substrate to use.
- A
- Through hole
- B
- Substrate
- C
- Copper foil
Multi-layer board
Multi-layer boards laminate copper foil and insulating layers called prepreg. This type is called a four-layer board, six-layer board, or eight-layer board according to the number of laminated layers. The more layers a board has, the more complex its structure. The designing and manufacturing costs increase accordingly. At the same time, multi-layer boards can contain power circuits and general signal lines between layers, increasing the surface area used for mounting and also increasing the mounting density.
- A
- Through hole
- B
- Prepreg (insulating layers)
- C
- Substrate
- D
- Copper foil
PCB Mounting Methods
PCB mounting is a process to solder electronic components on a PWB to function as a PCB. Two methods are commonly used to mount electronic components on PWBs: insertion mount technology and surface-mount technology. The characteristics of each technology are introduced below with figures.
Insertion mount technology (IMT)
This technology solders leads or electrodes inserted into through holes on a PWB. Solder applied to the inside of the through hole slightly reduces the impedance at the joint. A disadvantage of this technology is that components arranged on the surface require a larger substrate, making it difficult to miniaturise the PCB.
The leads on the components to mount are straight and point down for insertion into through holes. These components are called dual in-line packages (DIPs).
Surface mount technology (SMT)
Surface mount technology (SMT) is now most commonly used for PCB mounting. Electronic components are mounted on solder applied to lands on the PWB surface and are bonded by heating in a furnace. Through holes are not used. This is called reflow soldering. Unlike IMT, leads or electrodes do not need to pass through PWBs, so SMT can arrange components on both sides of a PWB efficiently with high flexibility. SMT can mount a larger number of electronic components, making PCBs smaller and denser.
Components to mount on surfaces, which are called surface mount devices (SMDs), are packages having lead tips parallel to lands or have electrodes at both bottom edges or on their sides.
Observation and Measurement Examples of Through Holes and Lands on PWBs
Through holes and lands on PWBs play an important role in connecting components to PWBs and components to components in circuits. In the mounting process, various items need to be controlled, such as solder printing, solder bath conditions, and temperature profiles of reflow furnaces. Even with the most careful attention to each process and material, defective through holes and lands can cause PCB conduction failures or malfunctions.
Through holes and lands on PWBs have surface irregularities and reflection specific to copper foil. It is difficult to observe PWBs at an angle with optical microscopes partly because it takes a lot of time and effort and only a part of the surface irregularities can be brought into focus. It is also difficult to measure 3D shapes and dimensions of microscopic through holes and lands on very small PWBs.
KEYENCE’s VHX Series 4K digital microscope uses optical and observation systems that have achieved both high resolution and a large depth of field, solving various problems.
This microscope is equipped with various functions accessible with simple operations—such as smooth tilted observation, observation using fully focused 4K images captured using depth composition and other related functions, acquisition of high-contrast images, and highly accurate 3D measurement—thus improving the sophistication and efficiency of work in quality assurance, research, and development of PCBs and PWBs. Read on for an introduction to actual observation and measurement examples of through holes and lands on PWBs using the VHX Series.
Tilted observation of through holes
Copper plating peeled from the inside of through holes can cause contact failures. To observe deep holes with an optical microscope, it is necessary to repeat tilted observation on a sample fixed at an angle with a jig, which is difficult and takes a lot of time and effort.
The VHX Series 4K digital microscope uses a free-angle observation system with high-accuracy XYZ motorised stage to enable tilted observation with easy field-of-view alignment, rotation, and oblique axis motion. The target stays centred in the field of view even if the lens is tilted or rotated, which enables tilted observation smoothly and quickly using high-resolution 4K images.
Additionally, the depth composition function enables tilted observation of recessed areas, like the insides of through holes, using clear images that are fully focused throughout the field of view even at high magnifications.
The VHX Series can also determine difficult lighting conditions with simple operations and no adjustment. Using the Multi-lighting function, which obtains data of images automatically captured with omnidirectional lighting at the press of a button, the operator can start observation just by intuitively selecting the image suitable for the purpose.
The following image shows a tilted observation example of the inside of a through hole, which is difficult to illuminate, using a bright and clear image captured with a combination of ring illumination and backlighting. Even microscopic defects such as copper foil peeled from the hole surface can be closely observed.
Observation of surface irregularities on lands (Optical Shadow Effect Mode image)
There are subtle surface irregularities on copper-plated land surfaces. It is, however, difficult to clearly capture the surface conditions due to low contrast.
The VHX Series 4K digital microscope is equipped with Optical Shadow Effect Mode, which easily captures high-contrast images while eliminating the need for preparation such as vacuuming. The surface conditions can be closely observed and evaluated using an image emphasising land surface irregularities.
3D measurement and profile measurement of defective plating on lands
Defective lands on PWBs, such as due to peeled copper plating, can cause PCB malfunctions or problems in the component mounting process. However, it is difficult to measure the microscopic 3D shapes of lands using contact measuring instruments or optical microscopes.
The VHX Series 4K digital microscope can perform highly accurate 3D measurement with high-resolution observation images. Measured values of uneven shapes and 3D images can also be acquired by capturing subtle surface irregularities and roughness using only an image captured directly from above.
Operators can also measure profiles at desired locations by simply specifying locations with a mouse while watching the screen. Measured values at sub-micrometre levels can be acquired for surface irregularities from a 2D cross-section shape of a defective area, which makes it possible to quickly perform highly accurate analysis.
A 4K Digital Microscope That Strongly Supports the Research, Development, and Quality Assurance of PWBs and PCBs
The VHX Series 4K digital microscope can perform advanced magnification observation and highly accurate 3D measurement of through holes and lands on PWBs that are indispensable to achieving higher PCB reliability. This microscope can also seamlessly support the full series of operations right up to and including automatic report creation with a single unit. Various functions that are accessible with simple operations make difficult tasks easier, reducing the time required for these tasks and improving work efficiency.
The VHX Series, which can be used for various observations and measurements required in the electronic device industry, is equipped with many other functions not introduced here. For additional product info or inquiries, click the buttons below.
