As a traditional PCB assembly method, THT Assembly （thru-hole mounting） is accomplished by combining manual and automatic procedures. THT Assembly is time-consuming and expensive. Today, most THT parts are replaced by SMD parts. For SMD parts assembly, SMT Assembly will be applied.
SMT Assembly is time-saving and cheaper, that’s why it’s popular and widely used. Most boards will not only use one assembly technology, it has to mix THT Assembly and SMT Assembly, so it’s called Mixed PCB Assembly.
Procedures of THT Assembly
- Step 1: Component Placement – Professional engineering staff performs this step manually. Engineers must quickly and precisely place components in corresponding positions based on the PCB design files provided by the client. To ensure high-quality THT Assembly, component placement must adhere to regulations and operation standards of the THT Assembly (thru-hole mounting) process. They must, for example, clarify component polarity and orientation, prevent operating components from affecting ambient components, make completed component placement compatible with corresponding standards, and wear anti-static wristbands when working with static-sensitive components such as ICs.
- Step 2: Inspection & Rectification – After component placement is complete, the board is placed in a matching transport frame, where the board with components plugged in is automatically inspected to determine whether components are correctly placed. If problems with component placement are discovered, they can be resolved quickly as well. After all, this occurs prior to soldering in the PCBA process.
- Step 3: Wave Soldering – The THT Assembly components should now be soldered accurately onto the circuit board. The board moves slowly over a wave of liquid solder at a high temperature, approximately 500°F, in the wave soldering system. Following that, all leads or wire connections can be successfully obtained, allowing THT Assembly components to be securely attached to the board.
After the above three steps, THT Assembly (thru-hole Assembly) is finished. QC process will be started, and once poor soldering quality is detected, rework will be required.
Process of SMT Assembly Technology
SMT Assembly (Surface mounting) outperforms THT Assembly (thru-hole mounting) in terms of manufacturing efficiency because it features a completely automatic mounting SMT assembly process that includes solder paste printing, picks and place, and reflow soldering.
- Step 1: Solder Paste Printing -SMT Assembly process started- A solder paste printer is used to apply solder paste to the board. A template ensures that solder paste is accurately applied to the correct locations where components will be mounted, also known as a stencil or solder screen. Because solder paste printing quality is directly related to SMT Assembly quality, PCBA manufacturers focusing on high-quality products typically conduct inspections after solder paste printing using a solder paste inspector. This inspection ensures that printing adheres to regulations and standards. If defects are discovered during solder paste printing, the printing must be reworked or the solder paste must be washed off prior to the second printing.
- Step 2: Components Mounting – After exiting the solder paste printer, the PCB will be automatically sent to a pick-and-place machine, where components or ICs will be mounted on corresponding pads under the effect of solder paste tension. Components are mounted on PCB boards in the machine via component reels. Component reels, similar to film reels, rotate to provide parts to the machine, which quickly sticks parts to the board. This process is called SMT Assembly or SMT/SMD mounting.
- Step 3: Reflow Soldering – Once all of the components are in place, the board is passed through a 23-foot-long furnace. The solder paste liquefies when heated to 500°F. The SMD components are now securely attached to the board. Unit now, the SMT Assembly process is finished, and the boards’ soldering quality checking will be started before sending them to the following process.
Mixed PCB Assembly Technology
Electronic products are becoming increasingly complex as modern science and technology advance, necessitating more complex, integrated, and smaller-sized PCB boards. It is nearly impossible for PCBAs with only one type of component to participate.
Most boards contain both thru-hole and surface mount components, necessitating the collaboration of THT Assembly (thru-hole Assembly )and SMT Assembly (surface mount technology), it’s called Mixed PCB Assembly. Nonetheless, soldering is a complex process that is influenced by far too many variables. As a result, it is critical to better organize the sequence of THT Assembly (thru-hole technology) and SMD Assembly (surface mount technology).
PCBA using Mixed PCB Assembly should be performed in the following situations:
- Single-Side Mixed PCB Assembly: The following manufacturing procedure applies to single-side mixed PCB assembly: When only a small number of THT components are required in this type of assembly, hand soldering can be used instead of wave soldering. For large quantities, boards must go through wave soldering, which is time-saving and cost-saving.
- One Side SMT & One Side THT: This type of Mixed PCB assembly procedure requires a wave soldering fixture, which can protect SMD parts and let the leads of THT parts immerse into the melted tin.
- Double Side Mixed PCB Assembly: This type is complicated. For top side THT parts, it can go through a wave soldering oven with a fixture, but for the second, it must use hand solder. When a designer makes the such design, he must move the THT parts as many as possible to one side, so when do hand-solder the second side, less time and cost will be consumed.
PCB assembly is such a complicated and technical process that numerous elements must be carefully considered, and even minor changes can have a significant impact on cost and product quality. This article’s descriptions of the PCB assembly process focus solely on standard PCBA procedures and technologies. The practical manufacturing process is heavily influenced and determined by design files and customer specifications. As a result, customers must consider how to evaluate a dependable PCB assembler before placing their PCBA. If readers have more doubts about THT Assembly, SMT Assembly, and Mixed PCB Assembly, or want to know the cost of their PCB and PCB Assembly, they can send emails to RFQ@yingstarpcba.com. Yingstar Electronics will give them professional suggestions for cost reduction.
