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The Most Essential Facts about Surface Mount Technology (SMT)

Current Overview of SMT

Up to now, SMT is the most popular technology and technique in the industry of Printed Circuit Board Assembly (PCBA). Since its advent into market at the beginning of 1970s, SMT has become the main trend of modern electronic assembly industry, replacing wave soldering assembly that has to depend on manual insertion. This process has been regarded as the second revolution of electronic assembly technology. Put it in another way, SMT has been a global trend in international PCBA, leading to a large transformation of the whole electronic industry.

Moreover, SMT has pushed electronic components towards chip type, miniature, thinness, light weight, high reliability and multiple functions and it has been a symbol indicating a nation's scientific progress degree.

Technique and Attributes of SMT

SMT refers to a type of PCB assembly technology through which SMDs (Surface Mount Devices) are mounted on the surface of PCBs through some technique, equipment and materials coupled with soldering, cleaning and testing to complete assembly.

• SMT technique

SMT technique can be classified into different types according to soldering method and assembly method.

a. Based on soldering method, SMT technique can be classified into two categories: reflow soldering and wave soldering.

b. Based on assembly method, SMT technique can be classified into full-surface assembly, single-sided mix assembly and double-sided mix assembly.

Elements influencing soldering quality mainly include: PCB design, quality of solder (Sn63/Pb37), flux quality, degree of oxidation on soldered metal surface (component soldering termination, PCB soldering termination), techniques such as printing, mounting and soldering (suitable temperature curves), equipment and administrating.

Reflow soldering quality is influenced by the following elements: soldering paste quality, technological requirement for SMDs and technological requirement for setting reflow soldering temperature curve.

a. Influence of soldering paste quality on reflow soldering technique

According to a statistics, problems caused by printing technique account for 70% of all surface assembly quality issues without consideration of PCB design or quality of components and printed boards. In the process of printing, malposition, edge subsiding, adhesion and insufficient printing all belong to disqualification and PCBs with these defects have to receive rework. Specific inspection standards should be compatible with IPC-A-610C.

b. Technological requirement for SMDs

In order to obtain ideal mounting quality, technique has to meet the following requirements: accurate components, accurate positions and suitable pressure. Specific inspection standards should be compatible with IPC-A-610C.

c. Technological requirement for setting reflow soldering temperature curve

Temperature curve plays a critical role in determining soldering quality. Prior to 160°C, temperature rising rate should be controlled at 1 to 2°C per second. If temperature rises too quickly, on one hand, components and PCB tend to suffer heat too quickly, which tends to destroy components, leading to deformation of PCB. On the other hand, such a high solvent evaporation speed tends to cause metal powder spilled with solder ball generated. Generally, peak value of temperature is set to be higher than melting point of alloy by 30 to 40°C (For example, melting point of 63Sn/37Pb is at 183°C and peak value of temperature should be set to be at 215°C) and reflow time to be 60 to 90 seconds. A low peak value of temperature or a short reflow soldering time will possibly lead to incomplete soldering without generating a metal alloy layer with a certain thickness. In serious situations, solder paste even fails to be melted. On the contrary, too high a peak value of temperature or a long reflow soldering time will make metal alloy layer too thick with soldering point intensity badly influenced. Sometimes, components and printed circuit boards will be possibly destroyed.

• Attributes of SMT

As a traditional PCBA method, Through Hole Package Technology (THT) is a type of assembly technology through which pins of components are inserted into through-hole vias on PCBs and then the pins on the other side of PCBs are soldered. THT has the following attributes:

a. Soldering points are fixed and technology is relatively simple, allowing manual operation.

b. Large volume and high weight, difficult to implement double-sided assembly.

Nonetheless, compared with THT, SMT contains more advantages that are listed in the following table.

Developing Trend of SMT


FPT refers to a type of PCBA technology through which SMDs whose pin distance is within the range from 0.3 to 0.635mm and SMCs (Surface Mount Components) whose length times width is no more than 1.6mm*0.8mm are assembled on PCB. Fast progress of electronic technology on computer, communication and aerospace leads semiconductor ICs to increasingly higher density, SMC to smaller size and pin distance of SMD to increasingly narrower. Up to now, QFP whose pin distance is 0.635mm and 0.5mm has become a communication component that has been widely applied in industry and military electronic devices.

• Miniature, multiple pins and high density

SMCs will be developing towards miniature and large volume and they have developed to the specification of 01005. SMDs will be developing towards small volume, multiple pins and high density. For example, BGA that is being widely applied will be transformed into CSP. Application of FC will become increasingly more.

• Green, lead-free soldering technique

Lead, that is Pb, is a type of toxic metal, harmful both to people's health and natural environment. Conforming to the requirement of environmental protection, especially with the common agreement on ISO14000, most countries around the world ban the application of lead in soldering material, which calls for lead-free soldering. In 2004, Japan banned producing or selling electronic manufacturing equipment through lead soldering. In 2006, EU began prohibiting producing or selling electronic manufacturing equipment through lead soldering. lead-free soldering is such an inevitable developing trend that major PCB manufacturers are striving for this trend so as to manufacture more products compatible with environmental requirements. PCBCart has been caring about environmental protection and it has been fully certified by UL and RoHS so that more lead-free and green products are being manufactured and encouraged.

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