Q: Basic methods of PCB assembly: THM & SMT

W hen it comes to the PCB assembly, there are two type of components that can be placed on a printed circuit board (PCB). The first one is Through-Hole Mounting (THM) and the Surface Mount Technology (SMT). You can make a decision on which one to use, even more, you can use both two of it on your designed PCB. Double-sided reflow requires two reflows, and have you ever wondered which side should pass through the reflow oven first? In general, the side with small components on it will pass through first. First of all, because the first reflow will not cause too much deformation to the PCB, the solder paste printing is relatively accurate, and the probability of small parts dislocation is small. Secondly, during the second reflow, the small components are facing up and will not come off due to the high temperature and gravity. Both methods have its Pros & Cons. We will go through this topic in the future articles.

Q: What is the purpose of using Nickel in PCB surface finish ENIG?

First, let's take a look at the characteristics of nickel. In addition to anti-oxidation, nickel can enhance the following mechanical properties. 1. Tensile strength 2. Elongation 3. Hardness 4. Internal 5. Fatigue life 6. Hydrogen embrittlement What role does nickel play in ENIG? The main purpose of using nickel in surface finish ENIG is to avoid the migration and diffusion between Cu and Ag, and play role as a barrier layer and protect copper layer from oxidation, furthermore, the nickel can also prevent poor conductivity and poor solderability. And in order to smooth the PCB assembly process, components are sometimes plated with a layer of nickel as a barrier layer.

Q: What Is Pre-treatment Before Inner Layer Process?

What Is Pre-treatment Before Inner Layer Process? After cropping process to have PCB material ready as working size, the first step we will scrub the surface of material to clear contamination and conduct micro-etching to enhance the adhesion with dry film in the next process of PCB material surface. And to plus curing to dry material, all three minor processes are so-called pre-treatment.

Question 1

Basic methods of PCB assembly: THM & SMT

Accepted Answer

When it comes to the PCB assembly, there are two type of components that can be placed on a printed circuit board (PCB). The first one is Through-Hole Mounting (THM) and the Surface Mount Technology (SMT). You can make a decision on which one to use, even more, you can use both two of it on your designed PCB.

Double-sided reflow requires two reflows, and have you ever wondered which side should pass through the reflow oven first? In general, the side with small components on it will pass through first. First of all, because the first reflow will not cause too much deformation to the PCB, the solder paste printing is relatively accurate, and the probability of small parts dislocation is small. Secondly, during the second reflow, the small components are facing up and will not come off due to the high temperature and gravity.

Both methods have its Pros & Cons. We will go through this topic in the future articles.

Question 2

What is the purpose of using Nickel in PCB surface finish ENIG?

Accepted Answer

First, let's take a look at the characteristics of nickel.

In addition to anti-oxidation, nickel can enhance the following mechanical properties.

1. Tensile strength

2. Elongation

3. Hardness

4. Internal

5. Fatigue life

6. Hydrogen embrittlement

What role does nickel play in ENIG?

The main purpose of using nickel in surface finish ENIG is to avoid the migration and diffusion between Cu and Ag, and play role as a barrier layer and protect copper layer from oxidation, furthermore, the nickel can also prevent poor conductivity and poor solderability.

And in order to smooth the PCB assembly process, components are sometimes plated with a layer of nickel as a barrier layer.

Question 3

3 Key elements for PCBA(PCB Assembly) quotation

Accepted Answer

The advantage of having PCB produced and assembled at one vendor
Once the bare board is produced, various components such as IC, BGA and other Active/passive components will be mounted on the PCB. After the PCB is assembled, there’ll be EMS before the end product i.e. smart phone, computer etc. completes. Thus, it makes sense that the majority of PCB suppliers will provide turnkey solution which includes PCB production, component procurement, PCB assembly or even EMS service to attract clients. For clients, the advantage of having PCB produced and assembly at same PCB supplier will be quality control. WHY? After the PCB assembled, the vendor could conduct function test, ICT test and so on based on client’s request. If the testing result fails, the PCB supplier could conduct experiment try to find the root cause and even fix the problem. On the other hand, if PCB produced at one vendor and assembly conducted at another. Most of time it takes longer process to find the root cause because it involves various parties. Let alone to mention the hassle of shipping the board back to the supplier.

3 Key elements for PCBA(PCB Assembly) quotation
So ExPlus is one of PCB suppliers could provide client PCB and PCB assembly service. In previous article, we have mentioned about the required information for PCB quotation. Today, we’re discussing the information required for PCB Assembly quotation. We’ll divide into 3 sectors as below.

1st element of PCBA quotation – Bill of Material
First, the client shall provide the basic information of BOM a.k.a. bill of material. In the BOM list, it shall include complete part number, usage quantity for each component and mounting location. Especially the part number, it’s crucial to provide the full component number of original manufacturer. Incomplete part number not only prolongs the searching time but affects the component’s size, the working voltage/current of and working temperature.

2nd element of PCBA quotation – Gerber or product’s picture
Second, the Gerber file or product’s picture is also required. The purpose is to compare the component’s corresponding location and also evaluating work method.

3rd element of PCBA quotation – Special requirements
Last, the client shall inform the supplier is there any specific requirement for the project. If ICT or function test is required, the testing condition, equipment and tooling shall be provided to the suppler. Also, for certain active components such as IC, the IC manufacturer will request the client to share the end product’s details such as application, selling location, the current situation of product’s development, end customer, annual usage and so on.

Finally, don’t forget to share the required quantity or even the EAU.

Check below link for relevant articles
Some Ways to Produce PCBA from PCB
PCB quotation x Gerber file

Question 4

How is the inner layer produced?

Accepted Answer

The pre-treatment that we discussed in last article was mainly to enhance the adhesion to help following inner layer process.

The first step is dry film lamination. To attach trace image photoresist film with base material to protect needed copper on the PCB. Second step to have UL light exposure to polymerize the film. Then the final step is to develop the trace on the PCB and to do etching with alkaline solution to remove the unprotected copper.

After the inner layer is done, of course AOI inspection is conducted to check if the trace is applied well enough.

Question 5

The Gerber File of Printed Circuit Board (PCB)

Accepted Answer

The Gerber file is an open 2D vector image format. It is a standard file type used in the PCB industry to describe the layer of the Printed circuit board (PCB), e.g. copper layers, solder mask, legend…etc. For fabrication purpose, the Gerber files must be contained in one .zip or .rar archive with the following standard file extensions:

Extension	Layer
pcb.GTL	Top Layer
pcb.GTS	Top Solder
pcb.GTO	Top Overlay
pcb.GBL	Bottom Layer
pcb.GBS	Bottom Solder
pcb.GBO	Bottom Overlay
pcb.TXT	Drill file
pcb.GML/GKO	Outline
pcb.GL2	Inner layer 2 (for multi-layer PCB)
pcb.GL3	Inner layer 3 (for multi-layer PCB)

Missing any files of the above list will cause PCB supplier a huge challenge. Also note, for getting a quote from a PCB supplier, the more detailed Gerber file you provide, the more accurate quote you will get!

Want to learn more about the Gerber file format? Please read the below article.
☑ What format of Gerber are most commonly used in PCB industry?

Question 6

Brief Introduction Process of COB (Chip On Board)

Accepted Answer

The key technique of COB (Chip On Board) is Wire Bonding and Molding, which is the process to package exposed integrated circuit (IC) as electronic component. The integrated circuit (IC) to extend its I/O through trace of package by Wire Bonding, Flip Chip, or Tape Automatic Bonding (TAB).

The process of COB is as follows.

Rework
↓
Die In → Clear → Dispensing → Die Attach → Curing → Wire Bond → Testing → Coating → Curing → Visual Check → Testing → FQC → Packing → Delivery
↑
Rework

Environmental requirement of COB – Clean Room

The Clean Room class must be above class 100000. Normally the Clean Room class of IC package testing shall be class 10000 or even class 1000. Clean Room for Wafer Fabrication is even higher.

*Related Article:
What is COB (Chip On Board)? Introduce the evolution history of COB

Question 7

Where are printed circuit boards used? III

Accepted Answer

PCBs are used in many different sectors. We have introduced the applications in telecommunication, consumer electronics.

Today we are talking about some applications in medical devices, which are highly specialized, such as HDI PCB, Flex PCB, and Rigid-Flex PCB. The using of medical devices are getting higher and higher during COVID-19.

Monitors: Healthcare monitors such as heart rate, blood pressure, blood glucose monitors, and forehead thermometer require single or double sides PCB most.
Scanning Equipment: Such as CT, CAT, ultrasonic scanners, and X-Ray screen often use PCBs.
Internal Devices: Heart pacemakers and similar internal medical devices will require small, HDI, flex, or rigid-flex PCBs.

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

Question 8

Does PCB have expiry date? Or long shelf life?

Accepted Answer

From time to time, clients will ask us is there any suggestion if they’re going to assemble the PCB which have received years ago. Our advice to them is baking before assembly.

PCB Should stay in vacuum package before assembly
Once the PCB delivered to clients, it’ll be arranged for assembly after IQC. But if the PCB isn’t scheduled for assembly just yet, please keep the board as packed condition. The vacuum package could prevent the board from absorbing moisture and keep the PCB in good condition.

Golden Time for PCB assembly after packed

So If PCB is packed, under temperature 23 +/-2 C/RH. 50~60%, we suggest assembly within half a year for PCB with HASL surface finish.
For PCB with OSP and Immersion Silver surface finish, the ideal assembly time is within a month after packed.
And for PCB with Immersion Tin and Immersion Gold surface finish, our suggestion will be 2 months.

Usually, when PCB stores over 2 months, it’s best to bake before assembly.
If you’re interested in this topic, you might find below article interesting as well.

Why long stored PCB shall be baked before assembly.

bake pcb before assembly if stores over 2 months, suggested by ExPlus PCB

bake pcb before assembly if stores over 2 months, suggested by ExPlus PCB

Question 9

What Is Pre-treatment Before Inner Layer Process?

Accepted Answer

What Is Pre-treatment Before Inner Layer Process?

After cropping process to have PCB material ready as working size, the first step we will scrub the surface of material to clear contamination and conduct micro-etching to enhance the adhesion with dry film in the next process of PCB material surface. And to plus curing to dry material, all three minor processes are so-called pre-treatment.

Question 10

Brief Talk about PCB Materials

Accepted Answer

When it comes to basic PCB designing and producing, there are various choices for PCB material like FR and CEM series. Do you know how to pick the one that suits you well?

Here, we will list the basic differences between the common materials.

FR series – The most popular material.
FR stands for “Flame Retardants”, which is a glass fiber epoxy laminate material that meets the UL94v-0 fire standard set by UL. The differences between material FR series (such as FR-1, FR-2 and FR-4) are their thermal properties, heat resistance and electrical strength. FR-4 is suitable and widely used from single-layer to multi-layered PCBs.

CEM series – Low grade but widely used for single / double-layer PCB.
Such as CEM-1 and CEM-3. CEM-1 is composite material composed of woven glass fabric and paper core combined with epoxy resin. Widely used for single-layer PCBs; and CEM-3 is made of glass cloth and glass mat composite substrate material, which developed on the basis of FR-4. For double-layer PCBs, it is a great alternative to FR-4.

Comparison of prices for the above materials: FR-1 < FR-2 < CEM-1 < CEM-3 < FR-4

So far, the material FR-4 is still the best choice for PCB production due to its cost-efficiency and good characteristics. And CEM-1 is the first choice of LED lighting manufacturers, because it has achieved the best balance between heat dissipation performance and cost.

PCB Knowledge