Q: Talk about “Bow and Twist” of PCB from IPC-6012E

<img style="display: block; margin-left: auto; margin-right: auto;" src="https://www.explus.com.tw/upload_files/Knowledge/Based%20on%203-4-3%20%E2%80%9CBow%20and%20Twist%E2%80%9D%20in%20IPC-6012%2C%20what%E2%80%99s%20the%20acceptance%20criteria%20in%20terms%20of%20%20PCB%20warpage%20for%20printed%20circuit%20board%20that%20use%20for%20surface%20mount%20components.png" alt="" width="523" height="438" /> Defined in IPC-6012E 3.4.3 Bow and Twist , unless otherwise specified in the procurement document, when designed in accordance with IPC-2221, the printed board shall have a maximum bow and twist of 0.75% for printed boards that use surface mount components and 1.5% for all other printed boards. In other word, PCB warpage under 0.75% are accepted according to IPC standard. Can twisted or bowed board be corrected (bent back) manually? No, if bending the PCB manually might worsen PCB warpage. Does PCB mounting process be affected by PCB warpage? Basically, PCB assembly supplier refer to IPC-6012E standard as well, if client has special requirement, they will produce carrier to support the PCB during the process. Also, SMT equipment, in the MOUNTING section, must have a splint design on the conveying track. So if the warpage within standard, the PCB could be mounted without problem.

Question 1

Surface Finish Is the Connection between Copper PAD and SMT Solder

Accepted Answer

Similar to solder mask, however, surface finish does not protect copper trace but the exposed copper PAD after solder mask process. As the connection between copper PAD and SMT solder, a few common options and comparison are as below.

OSP (Organic Solder-ability Preservative)
Advantage: low cost / good flatness / good solder-ability
Disadvantage: short storage time / not good for second time reflow

Lead Free HASL (Hot Air Solder Level)
Advantage: better cost / good solder-ability / better storage time
Disadvantage: bad flatness / not good for small component

ENIG (Electro-less Nickel Immersion Gold)
Advantage: long storage time / good flatness / good for second time reflow
Disadvantage: high cost / less welding strength

Green ENIG (Immersion Gold)

Question 2

Skin Effect and Its Effect on PCB Impedance

Accepted Answer

Simply put, the Skin Effect is the significant factor that cause the signal loss. At low frequencies, the current density inside the conductor is uniform, and the current flowing in the center is the same as the current density at the edges. But when the frequency gradually increases, the flowing charges will gradually approach the edge, and even no current will pass through the middle.

What is the specific impact of the skin effect in the actual PCB routing? For short transmission circuits, the loss in transmission can ideally be ignored. However, in practical, especially circuits with long-distance transmission, the additional loss caused by the existence of transmission trace resistance should be significantly considered. Thus, for PCBs, especially with high speed and high frequency features, when design and adjust the trace width to meet the impedance value, should consider the Skin effect and avoid the EMC issues.

Question 3

The Cause and Solution When Having Air Bubbles During Potting

Accepted Answer

We has introduced in previous article about what is COB, what is process of COB, and bonding wire and its pulling force. In this article, we’re going to talk the problem about the air bubbles in epoxy under the production process. It is not allowed to have bubbles exist in epoxy. The bubbles in the epoxy floor not only affect the aspect of the board, but also could damage the stability of AI wire and gold wire from inner wire bounding.

The solution is as follows

When there are bubbles in the epoxy
C- Improper mixing of adhesive and hardener.
S- Careful mixing.

There is air remaining during the process of potting.
C- The speed of potting was too fast or having some problems in the production process.
S- The mechanization can offer quality stability. Additionally, we usually use small needle for production and rotate from exterior to inner, the direction can also be reversed.

The too high viscosity of epoxy.
C- The too high viscosity cause the lack of liquidity and the voids cannot be filled properly. The liquidity increasing happens at the beginning of curing. Firstly the epoxy flows to fill the voids but bubbles are formed on the surface; secondly the air still cannot overflow completely and internal bubbles will be formed.
S- Find the epoxy which is low viscosity to increase the liquidity, or pre-heating the epoxy.

The inappropriate conditions of epoxy curing
C- The over temperature of curing cause the faster curing time, however, it also decrease the possibility of overflowing the bubbles from epoxy.
S- Do not increase the temperature to shorten the time of curing.

*Related Article:

COB1:What is COB (Chip On Board)? Introduce the Evolution History of COB
COB2:Brief Introduction Process of COB (Chip On Board)
COB3:Introduce COB's bonding wire and its pulling force

Question 4

Talk about “Bow and Twist” of PCB from IPC-6012E

Accepted Answer

Defined in IPC-6012E 3.4.3 Bow and Twist, unless otherwise specified in the procurement document, when designed in accordance with IPC-2221, the printed board shall have a maximum bow and twist of 0.75% for printed boards that use surface mount components and 1.5% for all other printed boards. In other word, PCB warpage under 0.75% are accepted according to IPC standard.

Can twisted or bowed board be corrected (bent back) manually?
No, if bending the PCB manually might worsen PCB warpage.

Does PCB mounting process be affected by PCB warpage?
Basically, PCB assembly supplier refer to IPC-6012E standard as well, if client has special requirement, they will produce carrier to support the PCB during the process. Also, SMT equipment, in the MOUNTING section, must have a splint design on the conveying track. So if the warpage within standard, the PCB could be mounted without problem.

Question 5

Solder Mask Is the Layer of Copper Protection

Accepted Answer

After copper plating and PTH process, outer layer trace is following as the next step. However, outer layer trace shares the same process of inner layer trace. For understanding more, please read below article.
https://www.explus.com.tw/faq/What-Are-The-Steps-of-PCB-Inner-Layer-Process.html

After all copper trace is done, we have to protect copper trace from oxidation. And that is what solder mask is printed on PCB for.

Similar to lamination, PCB manufacturer scrubs the PCB surface and then applies micro-etching to increase the adhesion when solder mask ink is printed on the surface. After printing solder mask ink, curing is applied to solidify the ink as the copper trace protection layer.

Green solder mask may be the most common color you can see on PCB. However, do you know what different colors of solder mask that are applied on PCB?

Question 6

What is impedance control? - Part.3

Accepted Answer

The impedance of the PCB trace will be determined by its resistance, inductance and conductance. The PCB impedance ranges from 25 to 120 ohm. The main factors that affect the impedance of the PCB traces are as below:

1. Trace width / thickness;
2. The dielectric constant;
3. Thickness of the medium;
4. The thickness of the pad;
5. The path of the ground line;
6. The traces around the trace

PCB transmission lines usually consist of traces, one or more reference layers, and insulating materials. Traces and slabs constitute the control impedance.

Question 7

Introduce COB's bonding wire and its pulling force

Accepted Answer

The previous article has introduced what COB is and the process of COB. Under the process, what kind of bonding wire and tensile test does COB have?
After the COB die is adhered and baked, the wire bonding process is followed. The COB process is different from IC packaging. It is gold wire for IC, and aluminum wire for COB. Therefore, the shape of the solder joint will be different, and secondly, the tensile force of the solder wire will also be different.

Wire Bonding

According to the shape of the solder joint, the wire bonding process can be divided into two types: "Ball Bond" and "Wedge Bond". COB usually uses aluminum wire (Al wire) so it will be Wedge Bond. However, according to experience and data, the strength of ball welding is better than that of wedge welding, but the "gold wire" is also more expensive.

Wire Pull Test

In the IC packaging process, the quality of the bonding wire is usually judged by three methods: Wire pull, die shift, and Ball Shift. Generally speaking, most COB The manufacturing process will only test the wire tension, which can measure whether the strength of the solder joints on the PCB is sufficient. The COB wire tension requirement is usually only greater than 3g according to the IPC specification (TM-650 2.4.42.3).

Question 8

Where are printed circuit boards used? VI

Accepted Answer

PCBs are used in many different sectors. Today we are talking about some applications in industrial products. PCB are widely used in the industrial part.

The PCBs used in the industrial sector often need to be especially high-powered and be durable enough to withstand the harsh conditions that exist in industrial facilities. PCBs may need to be resistant to rough handling, vibrating machinery, extreme temperatures or harsh chemicals. To meet this need for durability, industrial PCBs may be made with durable metals or thermal-resistant materials and be thicker than other kinds of PCBs.

Manufacturing equipment
Power equipment
Measuring equipment

*Related Article:
Where are printed circuit boards used?
Where are printed circuit boards used? II
Where are printed circuit boards used? III
Where are printed circuit boards used? IV
Where are printed circuit boards used? V

Question 9

Panel Copper Plating → PTH (Plated Through Hole)

Accepted Answer

The step follows drilling hole is copper plating.
Except those drilling holes for tooling purpose, the rest of the drilling-hole wall will be plated with copper as for the purpose of connection of trace in different layers or for those DIP components.

By blocking the holes and areas with no needs of copper, copper plating is applied not only on the drilling-hole wall but also on the whole PCB.
For requirement of finished 70um, normally PCB vendor takes 35um material and do copper plating to be 70um.

Question 10

What is E-TEST?

Accepted Answer

How does PCB manufacturer ensure the bare PCB is within standard before delivery?

Once of tests it will be conducted is “E-test” which is also known as electricity test. The main purpose of E-test is to make sure there’s no open circuit and short circuit.

When it’s sample production, most of time the manufacturer will use “Flying Probe test” for E-TEST. Even though flying probe test is comparatively time consuming in terms of production efficiency but it makes sense especially the project is still on pilot phase.

Once the project enters Mass Production, usually the PCB manufacturer will produce specialized tooling for each project. No matter it’s “Flying probe test” or “tooling test”, the main target is to detect NG circuit during “E-TEST STAGE” and ensure the quality is within client’s standard.

What is HiPot test?

PCB Knowledge