Q: Purpose of Back drill For Printed Circuit Board

Purpose of Back drill For Printed Circuit Board Back drillis a drill process for plated hole (via) on the PCB, the main purpose of back drill process is to reduce the length of the stub and its capacitance effect by eliminating the excess copper in the plated hole (via) on the PCB. By eliminating the excess copper in the plated hole, the transmission characteristics of high-speed signals can be improved, further increasing the transmission speed of high-speed connectors. Back drill can be applied to any type of circuit board where stubs cause signal integrity degradation, with minimal design and layout considerations. Now we’re going through the advantages of Back drill below: Reduced deterministic jitter Lower bit error rate (BER) Less signal attenuation with improved impedance matching Increased channel bandwidth Increased data rates Reduced EMI radiation from the stubs Reduced excitation of resonance modes Reduced via-to-via crosstalk Aspect ratio can be neglected (in contrast to blind vias)

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

Purpose of Back drill For Printed Circuit Board

Accepted Answer

Purpose of Back drill For Printed Circuit Board

Back drillis a drill process for plated hole (via) on the PCB, the main purpose of back drill process is to reduce the length of the stub and its capacitance effect by eliminating the excess copper in the plated hole (via) on the PCB.

By eliminating the excess copper in the plated hole, the transmission characteristics of high-speed signals can be improved, further increasing the transmission speed of high-speed connectors.

Back drill can be applied to any type of circuit board where stubs cause signal integrity degradation, with minimal design and layout considerations.

Now we’re going through the advantages of Back drill below:

Reduced deterministic jitter
Lower bit error rate (BER)
Less signal attenuation with improved impedance matching
Increased channel bandwidth
Increased data rates Reduced EMI radiation from the stubs
Reduced excitation of resonance modes
Reduced via-to-via crosstalk
Aspect ratio can be neglected (in contrast to blind vias)

Question 2

The design requirement of COB

Accepted Answer

In the previous article, we have introduced what is the COB process, wire bonding, and wire pull test. In this article, we’re going to talk about the design requirements of COB.

1.The surface finish should be either gold plating or ENIG to provide the requirement of Die Bonding, and the thickness of gold plating is usually larger than general.

2. The consistency of length of wire should be considered. In other words, to spread the risk of short circuit, the manufacturer have to keep the same distance from solder joint to Die and PAD. Shorten the spacing can avoid the appearance of diagonal PADs, and designing elliptical PAD also be helpful to equally divide the bonding wires into the relative positions.

3. Additionally, one wafer should have more than two positioning points, and replace the round hole with a cross shape. Wire Bonder is automatic-control, due to the restriction of the machine, the engineer should consider the function while setting the positioning points.

4. The Die Pad of the PCB should be bigger than the actual size, not only can control the shift of the die but also avoid the over-revolve in the die pad. The suggestion is every die pad on the board should be larger than the actual size.

5. It would be better not set vias in the potting area, or require the PCB manufacturer to plug those vias completely. The purpose is to avoid epoxy filters with the other side of the board.

6. For the convenience of dispensing and dispensing control, it is suggest to print the silkscreen on the areas which need epoxy.

*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 3

What Is Solderability Test and Its Purpose?

Accepted Answer

To ensure the reliability of PCB, PCB manufacturer will conduct multiple tests. One of destructive testing is Solderability test. It’s essential that there’s no delamination after Solderability test.

Testing method might slightly vary from manufacturer to manufacturer. Refer to IPC-TM 650 Solder float experiment, the operator will float the specimen for 4~5 seconds on the surface of a solder bath maintained at 260 +/- 5°C. Or the operator will dip the specimen for 10 seconds in a solder bath maintained at 288 +/- 5°C. This operation shall be repeated 2~3 times. The 2nd method called Edge dip test. Both methods could test the reliability of PCB and ensure the quality of Printed Circuit Board.

⇣⇣⇣ For more PCB tests before delivery. ⇣⇣⇣
What is E-Test?
What is Hi-Pot Test?

Question 4

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 5

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 6

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 7

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 8

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 9

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 10

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).

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