^{Step1:  Solder Paste Printing}

Solder paste printing is the begining of the SMT assembly process, the stencil will be installed on a solder paste printing machine, it is for accurately placing the solderpaste on our PCB solderpads.

The stencil was customized cutting according to the PCB solderpaste layer file and totally matching with the solderpaste on finished PCB. When in production, there is a rail for PCB moving from the begining till the end of SMT process, when the PCB reach the position under stencil, stencil machine will fix the PCB with stencil tightenly, and then printing machine will control a squeegee moving stable above on the stencil, it is making the solderpaste fully filled with vias on stencil. After seperated the PCB and stencil, we will get all SMT solderpads covering with neatly solderpaste efficiently.

Tips: We can control the solderpaste on PCB pads by adjusting the stencil thickness.

Solderpaste printing the most crucial process of SMT assembly, becuase solderpaste is the only raw material of PCB assembly process, almost 60% defective issues was caused by solderpaste in no good condition or not been placing properly. The quality causing by solderpaste usually is about more or lesser solderpaste was placing, or was placed not accurately, which directly causing the short-circuit, fake soldering or components joint soldering issues...ect.

Step 2: Soder Paste Inspection - 3D SPI

Since the solder paste is so important for the quality of PCB Assembly, there must require quality checking step before the PCB gose into next procedure, there is a machine called 3D SPI(3D solderpaste inspector) to inspect the solderpaste quality on PCB pads.

The working principle of SPI devices is to use 3D optical scanning technology to accurately measure the height, volume, area and position of solder paste on each pad and compare it with a preset standard.

With this technology, 3D SPI machine can help with detect the solder paste printing defects efficiently at before the components was placed on the PCB, which could help with avoiding a lot rework  time and labor cost.

Step 3: Pick and Place

The Pick and Place (P&P Assembly) process is the step of how electronics components were assembly on boards, this step was completed by high speed and accurate machines. Electornics components was taken out from the reels of SMT feeder, and were placed precisely on the position that was already designed by the PCB engineer.

You must be wondering how is the machine know where to place the right components, the working principle of a pick-and-place machine is very interesting.  When the PCB designers create production files, they will export a Pick and place file which enable the pick-and-place machines to recognize the X and Y axis coordinates of components. The machine is equipped with a vacuum nozzle that can quickly pick up components from the reel and precisely place them in their corresponding positions using pre-set coordinate axes and mark points.

In this process, it is all automatically and fast speed, thousands PCBA can be assembly within 1 working day on each production line. But the machines quality may effect the production efficiency and quality, such as wrong placement and wrong component orientation, could causing huge problems if mistake happens on one of the board, the whole batch could be the same problem.So it is very important for us to have the first initial board check after assembly the first piece. In Zero Mistake Solutions, IPQC(Initial Pieces Quality Control) is a very crucial step for avoiding massive mistake before we run the rest production.

Video: How is the Pick and place machine work

Step 4: Reflow Soldering

Reflow soldering is a step to solid the solderpaste by a high tempreture oven to make the components strong connecting with the PCB.

Solder paste, as a primary raw material in the PCBA manufacturing process, exhibits different states at varying temperatures: at room temperature, it is a viscous paste that can adhere components to the PCB. When the temperature reaches the melting point of the solder paste (for instance, high-temperature solder paste typically melts above 240°C), it becomes wetting and can fully cover the component leads and pads, forming a stable connection. After complete melting and subsequent cooling, the solder paste establishes a reliable and robust connection between the leads and pads.

Therefore, reflow soldering is essentially equipment designed to accommodate batch melting while ensuring a stable temperature process, based on the temperature-dependent characteristics of solder paste. The more stable the temperature curve inside the reflow oven, the more consistent the transformation of the solder paste as the PCBA passes through. If the oven temperature fluctuates unpredictably, it may not only prevent the solder paste from fully melting, leading to cold solder joints, but also risk damaging expensive components due to poor temperature control.

Hence, the temperature zones of a reflow oven are a critical factor in determining its quality. The more temperature zones there are, the smoother and more gradual the temperature rise and fall. This steady and slow temperature change helps prevent quality issues such as cracking, tombstoning, solder balling, cold solder joints, and warping, which can occur due to rapid temperature fluctuations.

Step 5: Quality check and rework

After the PCB passes through the reflow soldering process, the soldering steps of the PCBA are essentially completed. The next step involves conducting corresponding inspections of the soldering quality and carrying out rework on any identified issues. Common inspection methods include:

Visual Inspection: Manual, visual examination of the assembled PCBA to check for soldering quality and component orientation.

AOI (Automated Optical Inspection): Utilizes optical principles, image processing, and comparison technologies to efficiently detect defects such as incorrect components, missing components, misalignment, and solder bridges.

X-Ray Inspection: Used to inspect the internal soldering quality of components with hidden joints, such as BGA (Ball Grid Array) packages, which are not visible to the naked eye.

ICT/FCT (Electrical Testing): Tests the circuit board's electrical connectivity and overall functionality to ensure proper operation.

Rework: For defective units identified during inspection, operators perform repairs using soldering irons or rework stations.

Suggestions to new buyer: How to asking good questions for SMT Assembly supplier?

Understanding the SMT process will not only enable you to communicate more professionally with suppliers but also help you control quality at the source. 

You can try asking suppliers the following questions, which will make you appear well-informed, such as:

"Does your production line include SPI (Solder Paste Inspection) equipment?" – This reflects the supplier's emphasis on early-stage quality prevention.

"For boards with BGA components, do you provide X-Ray inspection reports?" – This is key to ensuring the quality of hidden solder joints.

"Please share a reflow soldering temperature profile." – The stability of the soldering process is demonstrated through data.

We hope this step-by-step explanation helps you build a clear understanding of the SMT assembly process. If you have further questions about any specific step or process selection for different board types, Zero Mistake Solutions Team would be happy to provide additional clarification.