Question 1

What are the purpose and application of rigid-flex PCB?

Accepted Answer

Rigid-flex PCB is a special flexible PCB composed of rigid PCB and flexible PCB laminated together, with a compact structure, and metalized holes to form electrical connections. It has the characteristics of high density, thin traces, small aperture, small size, light weight, and high reliability. It is still stable under vibration, impact, and humid environments.
Rigid-flex PCB has a bendable and three-dimensional installation, which effective use of installation space, is widely used in mobile phones, digital cameras, digital video cameras and other portable digital products.
Advantages of rigid-flex PCB,
1. Save the process of using connectors or soldering Hot Bar.
2. Shorten the signal transmission distance of the rigid-flex PCB and the speed is increased, which can effectively improve the reliability.
3. Simplify product assembly and save assembly working hours.
Disadvantages of rigid-flex PCB,
1. In terms of the unit price of “rigid-flex PCB” will be more expensive than “flexible board + rigid board”, but if adding the price of the connector or the cost of hot bar soldering, Prices may tend to be consistent.
2. SMT and reflow oven may need carriers to hold the flex PCB, which increases the SMT assembly cost.

What are the purpose and application of rigid-flex PCB

Question 2

What is aluminum boards (MCPCB), and what is its application?

Accepted Answer

Aluminum board(MCPCB) is a unique metal-based copper clad laminate. MCPCB has good thermal conductivity, electrical insulation performance and mechanical processing performance. MCPCB can reduce temperature, improve product power density and reliability, and extend product life.
MCPCB can reduce the volume, the cost of hardware and assembly. MCPCB is a metal circuit board material, which is composed of copper foil, a thermally conductive insulating layer and a metal board. Its structure is divided into three layers, circuit layer, insulating layer, and base layer.
There are three main material numbers for MCPCB, 1050: aluminum with a purity of 99.5%; 5052: Magnesium content is about 2.2~2.8%; 6061: Magnesium content is about 0.8~1.2%. 5052 is commonly used for MCPCB because it has good forming performance, corrosion resistance, and durability.
MCPCB also has the following unique advantages: it meets RoHs requirements; it is more suitable for SMT process.
The main application of MCPCB: often used in high-temperature electrical products or for heat dissipation requirements, such as LEDs, high-power electric lamps, etc., in addition to power supply equipment, audio equipment, automobiles and other applications.
The tests that MCPCB will do are open short and hi-pot tests because the European regulations of lamps and lanterns clearly require that lamps and lanterns have withstand voltage requirements, our company will do 100% of the hi-pot tests.

Question 3

What in the world are the major causes of this PCB bending issue?

Accepted Answer

There are various reasons the cause the PCB bending issue. The most common one is when the stress applied to the PCB is greater than the stress which PCB itself can withstand, the PCB bending issue occurs right immediately.
When PCBs go through the reflow oven for SMT process, the major source stress is the “high temperature”. High temperature will soften the PCBs, plus the factor of Coefficient of Thermal Expansion (CTE) and the characteristic of thermal expansion and contraction of PCB, if the temperature reaches the upper limit of the TG value of the PCBs, than the PCBs will bend.
The PCB designers rarely consider whether there is copper on the PCB edge. On the circumstances of without copper on the PCB edge, the PCB will easily bend while high temperature.
Also if the size of PCB is relatively big and the SMD components are heavy. The PCB bending issue will occur because of the weight.
The V-cut designed for conveniently separating the PCB will also destroy the structure of the PCB, and easily cause the PCB bending issue.

Question 4

What are the differences between ED copper and RA copper in flexible boards (FPC) applications?

Accepted Answer

With more people opt for small electronic devices, it has increased the needs of Flexible boards(FPC) applications for PCB manufacturer. Understanding the differences between ED copper and RA copper, will help PCB designers to choose suitable printed circuit board materials in order to prolong the lifetime of PCB products.

Both ED copper and RA copper are a type of copper foil for flexible boards(FPC). They are a layer of conductive attached to the outer surface of the non-conductive substrate.
During PCB manufacturing process, the ED copper that uses in flexible boards(FPC) is made by using electrolysis method, its crystals tend to grow lengthwise, with long crystal boundaries perpendicular which makes it easy to break. Whereas the RA copper is made by rolling operation, its crystals are smaller and have spherical shapes which give it great extension.
Few tips when choosing the flexible boards(FPC) materials. RA copper has weaker conductivity than ED copper, however, it has much better extensibility, which can reach to 20-45%. RA copper is great for flexible boards(FPC) that needs to bend frequently.
As for PCB products that has less bending requirement, for example: keypads, module boards. The ED copper could be a choice with it great conductivity and lower cost for PCB manufacturers. On the contrary, RA copper will be an excellent choice for bendable boards such as flip phone, Zoom optical lenses.

Question 5

Why are PCBs usually in green color?

Accepted Answer

Solder mask is a protective layer of polymer. It function as a mask layer by forming a protective coating on PCB. The main purpose is for preventing copper trace on PCB from contamination. Furthermore, by covering solder mask on copper trace could prevent broken trace. (Some exposed pads might risk falling out and solder mask could minimize the situation). The solder mask is moisture proof and can provide insulation, resistance, heat endurance and to maintain appearance.
Solder mask, also known as solder resist. Though 99% of PCB use green solder mask, there are other solder mask color available for consideration, such as red, black, blue and so on.
The most common used PCB solder mask is made of Epoxy Liquid. It is typically applied using silkscreen techniques onto the top and bottom side of PCB, where the soldering is not required. In some occasions, the PCB manufacturers might apply inkjet printing technology.
In general, most of PCB manufacturers would apply the lacquer on entire PCB and then put in the oven during pre-baking process and then exposure the film by using contact-exposure technology to transfer the image onto the solder mask layer.
The process of solder mask seems fairly easy. However, if the solder mask is not applied properly during PCB manufacturing process, it might cause some quality issues, such as contamination, skip printing, copper exposure, unequal printing etc. Moreover, the thickness and displacement of solder mask could have affected the quality of tin soldering during SMT.

Question 6

What is the role of holes on printed circuit boards (PCB)? What are PTH / NPTH holes? And what’s the difference between two of them?

Accepted Answer

If you look closely at the printed circuit boards (PCB). You will see that there are many holes with different size on the boards. Those holes on PCB can be simply divided into two types: PTH (Plating Through Hole) and NPTH (Non Plating Through Hole).
The easiest way to distinguish between PTH (Plating Through Hole) and NPTH (Non Plating Through Hole) in PCB is to see if there are any traces of plating on the borehole wall in the PCB.
NPTH (Non Plating Through Hole) refers to a hole without copper in the borehole wall. It is generally used as the positioning hole and screw hole of PCB. The hole diameter is usually larger than PTH.
PTH (Plating Through Hole), the borehole wall has a layer of copper, the PTH holes on PCB has two purposes:
1. Used as component hole for welding DIP components, the hole diameter must be larger than the pin of the component, so that the component can be inserted in the PTH.
2. Another relatively small PTH (Plating Through Hole), also known as “Via”, can be used to connect and conduct wiring between different layers in PCB.

Question 7

Some Ways to Produce PCBA from PCB

Accepted Answer

PCB production is a complicated process, especially PCBA. There are some ways to produce PCBA from PCB, such as SMT, DIP, etc.

First, we will introduce how to produce PCBA. PCBA means Printed Circuit Board + Assembly. That is to say, the SMT or DIP process for PCB bare board, is called PCBA.
SMT and DIP are both the process that put the components on the PCB. The main difference is that it is no need to drill holes for SMT; but DIP needs to put the components’ PIN into the drilling holes.
DIP, plug-in component, which means to plug in the component on the PCB. Due to the larger dimension of the component, when it cannot use mount or SMT, it will use DIP. There are manual plug-in and automotive plug-in currently.
The main process for DIP is adhesive pasting (Prevent tin plating from undesirable places), component plug, inspection, wave soldering, board brushing(remove stain caused during wave soldering), and production inspection
DIP is also known as dual in-line package. The shape of the integrated circuit is rectangular, and there are two rows of parallel metal pins on both sides, called pin headers. The components of the DIP package can be soldered into the plated through holes on PCB, or inserted into the DIP socket.

Question 8

How to choose the material of PCB?

Accepted Answer

Selecting a suitable printed circuit board materials is one of the most important parts during PCB manufacturing. To understand the differences of printed circuit board materials will help you to better choose a suitable PCB material for your products in order to optimize the performance of the application.

The printed circuit board materials are typically comprised of epoxy resin, glass fiber (reinforcement) with a copper foil.
The type and the amount of resin contain will affect the printed circuit board materials properties. Therefore, it’s better to select the printed circuit board materials that align with the desired PCB design.
Currently, the most common printed circuit board materials for the substrate of a PCB is FR4, a flame retardant grade.
The following are the most common printed circuit board materials that used in PCB manufacturing industry.
FR4: Glass fiber + epoxy. (The most commonly used printed circuit board materials), High TG FR-4, CEM-1, CEM-3 etc.
There are some elements that need to take into consideration while choosing the printed circuit board materials. For example, the required TG for final products, the rate of CTE, the flatness of PCB and heat endurance during the SMT process (risks of temperature rising).

Question 9

What does the term “X-out” mean in the PCB industry?

Accepted Answer

In the PCB industry, the term “X-out” also known as “Cross-board” and “X-board”. It stands for the defective boards in PCB panels, and X marks are usually marked on the top.
Normally, the SMT factory will not allow X-out boards appear in PCB panels, because it cause the loss of efficiency during the process of SMT. However, the X-out boards is inevitable while producing PCBs, the more PCBs in one single panel, the more likely X-out boards appear.
The Acceptable Quality Level (AQL) for X-out boards in the PCB industry, generally, the number of X-out boards must be less than 20% of the number of PCBs in a panel. That is, in a panel with 4 pieces of PCBs, only allowed 1 X-out board in it. In a panel with 8 pieces of PCBs, only allowed 2 X-out boards. And the total amount of X-out boards must be less than 10% of total PCB shipments.
When about to ship the PCBs, must be assured that in each vacuum bag, the position of X-out boards must be the same position in each panel. If the X-out boards less than the quantity of the whole package, it must be packed in the mantissa box, and clearly marked with the total quantity of PCBs and X-out boards and relevant information.
If the buyers allow X-out boards to exist in PCB panels, the SMT factory can skip the X-out boards without doing SMT process by modifying the SMT program or by identify the bad marks on PCBs. But it will cause the SMT efficiency loss.
If the buyers will NOT allow X-out boards to exist, it means the PCB manufacturers are forced to produce more PCB panels, in order to eliminate the PCB panels, which has X-out boards in it. Therefore, the purchase cost will increase, but the SMT process will achieve the maximum of efficiency.

Question 10

The pro and con of the gold finger and its application on the PCB.

Accepted Answer

Gold finger is the metal connector on the PCB, and gold finger is also known as Gold tips or Gold tabs.
Gold fingers are the gold plated connectors on the edge of PCB that arranged like fingers. During PCB manufacture process, gold finger will be coated with a layer of gold through a special process on the copper cad lamination. However, today, many PCB designer has replacing the surface finish of gold finger by tin plating.
For PCB designer, the aim of gold finger is to connect the PCB by compressing the corresponding slots with connector, which allows its to communicate with one device to another.
The advantages of using plating gold on gold fingers for the PCB: The gold finger can resist oxidation and it is durable with high conductivity.
The disadvantages of gold finger on the PCB: it has relatively uneven surface, the needs of connecting line and the price of gold finger is extremely expensive.
The difference between applying immersion gold and plating gold on the gold fingers is that immersion gold is easier to solder compare to plating gold and will not affect the signal, it is also hard to get oxidized. However the immersion gold is less durable.

Knowledge