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Determining SMT Process and Its Packaging

by: Nov 01,2018 106 Views 0 Comments Posted in PCB Assembly

Abstract:

Standardized design is good for the accomplishment of electric with SMT and the improvement of production efficiency. Introduce the standardized design of process flow, the choice of components, the layout of PCB, the requirement of SMT machine for design. The standardized design will assure the products to be of higher manufacturability, quality and production efficiency.

Determine the Technological Process:

In the initial stage of product design, the designer should basically determine the technological process of the product according to the requirements of product planning. If the product design is not targeted and messy, the system cannot be maintained well. SMT assembly process is according to the product components installed for use in this situation, the order quantity of product, with the auxiliary technology jig condition such as the matching ability and use of equipment technical parameters, to ensure product quality, shorten the production cycle and reduce the production cost as the objective and comprehensive consideration of the production process. The production process adopted by most enterprises is as follows:

1.    Single-side PCB:

a)      Solder paste printing → Components mounting → Reflow soldering → Inspection and repair

b)     Patch adhesive coating or printing → Components mounting → Adhesive curing → Wave soldering → Inspection and repair

Process a) is a commonly used process, characterized by simple process and convenient quality control. Process b) is not suitable for those PCBs containing the integrated circuits with the pin spacing less than 0.8mm, which is characterized by the ability to mix with THT components for wave soldering at the same time.

2.    Double-side PCB:

A-side of template printing solder paste → A-side of template mounting components → Reflow soldering → Inspecting A-side of template → Flip PCB → B-side of template mounting components →Reflow soldering → Inspecting and repairing A-side and B-side of template

If there are no too many plug-ins, wave soldering should not be used. The A-side components should only be selected for simple small chip and small mass components. If there is high quality IC on both sides, the welding material with different melting point temperature will be used. It is difficult to carry out the process and the manufacturing cost will be increased.

3.    SMT is mixed with THT:

a)      Patch adhesive coating or printing → Components mounting → Adhesive curing → Flip PCB and plug-ins → Wave soldering → Inspection and repair

b)     Template printing solder paste → Components mounting → Reflow soldering → Plug-ins → Wave soldering → Inspection and repair

c)      Template printing solder paste → Components mounting → Reflow soldering →Flip PCB → Patch adhesive coating or printing → Components mounting → Adhesive curing → Flip PCB → Wave soldering → Inspection and repair

PS: SMT components and THT components can only adopt process b) when they are on the same side. At present, most products also adopt this process. In addition, some products have high installation density and complex process, so it is extremely difficult to adopt the fully automatic production process, so manual welding operation should be combined to a certain extent.

Selection and Packaging of Components & Packing Form Requirement:

In PCB design, standard surface mount components should be selected as far as possible to reduce product size and facilitate large-scale production. Firstly, components suitable for SMT technology production should be selected. When selecting components, common components and standard encapsulated components should be selected as far as possible on the premise of meeting the design requirements. On the premise of ensuring product function realization, we should try to choose components of the same size for high-speed production. The pin coplanability of the components shall not be greater than 0.1 mm, and the heat resistance, the thermal shock, the thermal stress and cleaning resistance of these components can meet the requirements which generally include that the temperature should be between (215+-20) and 20 to 60 seconds, and the wave soldering (260+-20) is 10 seconds. It is also necessary to ensure that the pins of the components are well weldable and are not easily oxidized or deformed. At the same time, the packaging form and size tolerance of components should conform to the standard. In general, braided packaging should be adopted as far as possible. In this way, components are not easy to oxidize, and the material supply is convenient and fast during mounting.

 


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