SMT PCB Assembly Process - Surface Mount Technology

In the fast-paced world of electronics manufacturing, surface mount technology (SMT) assembly is a pivotal innovation that has reshaped the way we produce printed circuit boards (PCBs). This article will take you on an in-depth look at the SMT assembly process as well as the pros and cons!

What is SMT assembly?

SMT, its full name is "surface-mount technology". SMT assembly is the method to place and solder the electronic components on the surface of PCB correctly by using automated machine. With the development of smart technology today, SMT assembly has replaced traditional through-holes technology construction method of fitting electronic components, SMT assembly allows to increase manufacturing automation so that it reduce the manufacturing cost of PCBs greatly and lead to smaller board.

What is the advantages of SMT assembly?

1. High density SMT assembly: Based on technology development, electronic products becomes smarter and smarter, more and more sophisticated, it required that PCB assembly density need to be greatly improved. And SMT assembly solved this problem perfectly, it makes high density PCB assembly become possible.

2. Lower cost and faster produce speed: due to the features of standardization, automation and holeless mounting, smaller-size components mounting reduce a lot of cost than their larger through-hole PCB, reduce drilling and increase the speed of produce.

3. Higher performance: Using electronic components with short-lead or no-lead, SMT assembly reduce parasitic inductance and capacitance from the leads, improves the frequency and speed performance of PCB. Keep the PCB and components out of heat effectively.

4. Reliable and stable: automated production machines ensures that every connection of the components are soldered well, SMT assembly improve the reliability and stability of electronic products.

5.More efficient use of pcb area: smaller electronic components and the technology of SMT allow SMT assembly to have a better use on the PCB's surface area.

6.SMT assembly helps PCBs circuits faster

What processes does SMT assembly have?

Normally, it has 16 processes in our PCBasic company.

Components purchase -- IQC -- PCB fabrication -- Stencil preparation -- Program SMT assembly machines -- Feed preparation -- Solder paste printing -- SPI -- SMT assembly -- Inspection -- Reflow soldering -- AOI and Visual inspection -- X-ray inspection -- SMT assembly QA -- Anti-static packaging

How do every process work?

Firstly, purchase the electronic components in need, IQC ( income quality control ) ensures every component is in good quality and less errors in feeding.

Secondly, Put them into Smart material management system, every material has its own unique QR code to make sure projects produced correctly. When the project starts, we will scan its QR code to get the correct quantity and type of electronic components in need. It helps SMT assembly process will place the correct electronic components on the PCBs.

3rd step: PCB fabrication. Produce the PCB following the files of PCB requests. Make sure that every components have correct pads on the correct places.

4th step: Stencil preparation, follow your pcb assembly files to make Stencil for the SMT assembly process, we use a Laser Printer to dig holes on the stencils in order to fit the pad on PCBs. it will help the machines

print the solder paste on the PCBs' pads.

5th step: Program the SMT assembly machines to pick and place the electronic components on the PCB correctly.

6th step: Feed preparation. To get the electronic components from the smart management stock, put the feeds into SMT assembly machines by scan the QR code of each kind of electronic components, if scan the wrong QR code, the machines will give you an error. In this way, we have less errors than other manufacturers.

7th step: Solder paste printing, solder paste is the mixture of flux and tin, printing machines print the solder paste on the PCB  by using a squeegee. it is an important thing that design the thickness of stencil and adjust the pressure of the squeegee, the thickness of solder paste on the PCB depends on them. it will have a great influence on the next processes. So it need to be Scientific, refined and standardized. 

8th step: SPI ( solder paste inspection ), One of important  processes for SMT assembly. It is a machine to check the solder paste on the PCB, Solder paste inspection machine adds a laser device for solder paste thickness measurement, through checking Solder paste's printing volume,the height , Area, Flatness to make sure components soldered better.

9th step: Pick and place electronic components on PCBs through high-speed SMT assembly machines. Each feeder are able to pick and place the electronic components of 0201 and upward. During this step, SMT assembly machines pick the correct components and place them on the PCBs accurately following the program. This process enables fully automated assembly and most SMT assembly machines can place over 40k components per hour.

10th step: Inspection before reflow soldering, check the whether the machine printed solder paste well on the PCBs. if well, it will be sent to the next process, if not, PCBs will be sent back to last process until it pass the checking.

11th step: Reflow soldering. During this step, the point is to melt the solder paste and make the tin climb on the connections of components, then solidify. After the PCBs leave the SMT assembly machine, it will be sent to reflow oven. it has 10 different temperature zones, will heat the PCBs and solder paste. 

During hot gas soldering, the energy for heating up the solder joint is transmitted by a hot gas. This can be air or nitrogen. In the reflow oven, PCBs will be heated to about 235-255℃ which is higher than the melting point of solder paste. within the temperature, need to consider the heat resistance of components. Solder paste is melt after heating, and flux help the tin climb on the connections of the components and then solidify. 

12th step: the next step is AOI ( Automated Optical Inspection ) after reflow soldering, check if that is no problem with the solder joint quality. With the development of 3D technology, it is more reliable to use 3D technology to check it than 2D inspection. Cause there are more errors by 2D inspection, through 3D physical optical imaging capabilities, 3D inspection allows to take more accurate measurements and provide more inspection process. As the tin hardens, components are affixed on the PCBs, SMT assembly process is completed.

13th step: X-ray inspection. After the SMT assembly, if the pcb have BGA and other flat contacts, a matrix of solder balls or terminations on the body of the component. 

The size of the chip is also getting smaller and smaller, and the pins of the chip become more and more, especially the BGA chip that has appeared in recent years, because the pins of the BGA chip are not distributed around the conventional design, but are distributed on the bottom of the chip. 

There is no doubt that it is impossible to judge the quality of solder joints through traditional visual inspection. AOI can't inspect if the connections of components like BGA are soldered well, so it need other method to help inspection. X-ray equipment can detect it by using the relationship between the penetrating power of X-rays and the density of materials, and using the property of different absorption to distinguish materials with different densities. 

Therefore, if the inspected object is broken, the thickness is different, and the shape changes, the absorption of X-rays is different, and the resulting image is also different, so a differentiated black and white image can be generated.

14th step: Cleaning and Drying. There are some oil and dirt in the whole production process and they may lay on the surface of PCB, so PCB need to be cleaned and dried before next process. For example, soldering paste leaves behind some amount of flux, while human handling can transfer oils and dirt from fingers and clothing to the surface of the board.

15th step: SMT QA. it is the last step of the SMT assembly processes. QA( quality assurance ). In this step, we check the PCBs again and test the PCB to test the result of SMT assembly.

16th step: Anti-static packaging. Cause some electronic components are easily damaged by static electricity, preventing static electricity from them and Anti-static packaging are necessary.

No matter how perfect your manufacture factory it is, it still has some troubles in production. Some of these troubles are caused by nature, the others occur due to manufacturing errors. Surface Mount Technology Assembly (SMT assembly) is one of those processes which experiences such errors, but almost errors can be avoided.

Here are some common problems during SMT assembly.

1. Solder Bridging
This phenomenon is caused by bridging across two different components, then lead to electronic short. The reason is that when printing the solder paste, it generates excess solder on pcb and cause wrong soldering on the PCB pad. You could try to make stencil thinner to see whether difference happens.

2. Solder Balling
Due to excessive humidity in the SMT assembly device, it creates these solder balls during the reflow solder process. Those balls may lead to electronic disruptions and some big balls of them will cause functional issues in PCB. Thus, necessary to reduce the humidity and moisture in the devices and Please clean the bottom of the stencil as dirt may also bulid these solder balls.

3. Tombstoning
This error has another name:“Manhattan Effect". It happens when one connection lead of electronic components raise and much higher than another lead, looks like a tombstone on the ground. Thus, this phenomenon got this name. Uneven heat distribution of solder paste is one cause of warping. The other reason is that electronic components aren't placed at correct postions.

4. False soldering

It seems to be soldered on the surface, however, actually not soldered. And when you test its function, sometimes it can pass the test, sometimes not, it will show your its bad contact.

PCBasic Company

PCBaisc company, a global high-mix volume hig-speed PBCA manufacturer with 19 years PCB assembly experience, it equipped 8 SMT lines and 6 DIP lines in the factory. It guarantees the production speed and quality stability.  There has a powerful supply chain can help customers source electronic components at a reasonable price.

The company has passed ISO9001 / IATF16949 / ISO13485 quality certifications and won the national high-tech enterprise award, has over than 200 employees, Among them: over 30 people in engineering technology, 20 people in quality management, 10 people in IT and 2 employees in R&D, 24 hours support to provide customers professional PCBA production services. Welcome to contact us, please send your Gerber files and BOM list for quotation.