PCB Assembly

Common Challenges in SMT Circuit Board Assembly and Solutions

SMT circuit board assembly

What is SMT circuit board assembly?

SMT circuit board assembly is short for surface mount technology printed circuit board assembly. It means the electronic components are placed and soldered on the surface of PCB pads. Together with PTH (Plated Through Hole) assembly, SMT assembly is one of the two most important PCB assembly technologies.

PCB SMT circuit board assembly process is an automatic method of assembling electronic parts. It is an accurate and effective PCB assembly method. So, the SMT circuit board assembly process is more complex for high-complexity design.

It’s also very tough for the assembly production house due to the component’s small volume and many procedures that need to be followed. SMT circuit board assembly vendors are facing many issues like poor solder paste, bridging issues, etc. Sometimes component sizes are not easily handled with pick-and-place machines.

Common challenges of SMT circuit board assembly process and how to solve them:

  1. Rework
  2. Improper solder
  3. Testability
  4. Solder bridges
  5. Solder ball joints
  6. Wrong parts
  7. Tombstone
  8. Cold joints
  9. Signal integrity and power integrity
  10. Shadowing

Here is a brief explanation of all the common challenges of the SMT circuit board assembly process. 

SMT PCB assembly

1. Rework

Rework is the most important part of the PCB manufacturing process. Basically, it’s to repair and replace the faulty parts during quality inspection. Rework is the most challenging part of the  SMT circuit board assembly process. Because it’s a very complicated operation to remove parts and place them in the same location.

  • Solutions:

To solve this problem, we need to make sure whether the whole assembly parts are accurately placed or not and designers need to check SMT capabilities.

 

2. Improper solder

Sometimes solder pads are not enough for placing the part. So at that time the solder was improper. These solder joints are called bad solder joints. Sometimes paste is less applied to the part as per design. Then filets are generated on pad corners due to less paste applied. Solder stencil is the first stage of SMT circuit board assembly. Due to improper solder; electrical connections failed.

  • Solutions:

To solve the issue, we can ensure to paste at the earlier stage of SPI at DFA. So it suggests the customer as per IPC standards aspects ratio for solder pads for placing parts.

3. Testability

Testability means just testing a test point using flying probes. Sometimes designers would mess up the whole design by placing lots of SMT components near the test points. So, it’s very difficult to test the test point at the quality stage.

  • Solutions:

To solve the issue, please make sure all SMT components are placed in a manner way to test points easily and if not then suggest the customer change location for easy testability.

4. Solder Bridges

Solder bridges are easily seen on paste stencils. So that time we can hold a paste stencil to solve or raise a new stencil data from the customer. Sometimes a high amount of solder is applied to pads. It created electrical solder bridge connections between two pins of parts. And it’s shorting whole assembly data. It’s basically coming from improper paste masks, through-hole components of higher lengths, solder reflow profiles, etc. Sometimes solder bridges are intentionally provided by customers. So, it can be ignored.

  • Solutions:

To solve these issues, provide a solder mask properly, use fiducials, and follow them through hole parts length and SMT pad size according to the IPC standards.

 

5. Solder ball joints

A solder ball is no clean process. It provided a connection between High-quality chips and PCBs. In electronics, many chips and ICs have ball grids and long grid arrays. It needs too many handling and moisture requirements. Because they are very sensitive parts and it’s a challenging part of SMT manufacturing, don’t skip stencil inspection for solder ball joints. If the stencil is not accurate, then it’s a huge challenge for solder joints in the SMT circuit board assembly process. Solder ball joints occurred when the solder stencil was improper, reflow profile, oxidation, and alignment of solder paste stencil.

  • Solutions:

To solve them, we need to check the whole reflow profile, solder stencil adjustments, moisture precautions, etc.

6. Wrong parts

There are many situations for misalignment of parts on the PCB. It’s the major challenge of SMT circuit board assembly due to wrongly placed parts on the PCB. This error occurs when the reflow process and alignment are not accurately provided in the pick and place file. A wrong parts error occurred due to a polarity mismatch in the silkscreen on the PCB.

  • Solutions:

To solve this issue, make sure that the alignment is at the earlier DFA stage and avoid wrongly touching up for misalignment.

7. Tombstone

Tombstone is a major defect in the SMT circuit board assembly process. Sometimes, during the wet soldering process, a completely one-sided part is soldered and the other part is vertically opened. This type of error occurred majorly in MLCC components. It’s very small and toughest to rework them. Various types of sizes of pads increase thermal imbalances.

  • Solutions:

To solve this drawback, ensure the whole placement of electronics parts, quality inspection before the reflow profile, and improvement of pad sizes.

Cold solder joint

8. Cold joints

Cold joints are also known as grainy joints. Basically, solder is applied to the PCB. Then it goes to the reflow process for melting them. Some connections remain from the melting process and it’s the result of cold connections that are made on the PCB. It’s the major drawback of the SMT circuit board assembly process because it’s very difficult to clean due to many purities in that cold joint.

  • Solutions:

Proper soldering techniques should be used. Besides, pay attention to the parameters such as temperature, speed, density., etc.

 

9. Signal and power integrity

Signal integrity means that components are placed correctly but PCB performance is reduced due to signal integrity. To solve this signal integrity drawback PCB designers placed components that are well accurate with isolation and signal integration with proper schematic.

  • Solutions:

To reduce the integrity of the power supply, we need to move all bypass capacitors away from the supply pins. This will reduce noise and also increase the power performance on the PCB.

 

10. Shadowing

Many components are smaller on the PCB pads, which means that it does not need more solder to place. But sometimes higher solder applies to the smaller part and then goes to reflow. It plays a large part in wave soldering. So resultant it’s called al shadowing.

  • Solutions:

Proper solder stencils should be used, but the best way is to revise the PCB files to get correct PCB pads.

Conclusion:

After seeing the above drawbacks and solutions, we must create a PCB according to SMT circuit board assembly manufacturing standards and capabilities and IPC standards for design rules. Therefore, these drawbacks have not occurred in any assembly production house. We can’t skip those drawbacks and solutions for SMT circuit board assembly manufacturing processes.

Sometimes small components are more difficult to rework. It takes more time to rework that. The above drawbacks are the root causes of SMT production houses. SMT circuit board assembly processes are having many problems and many configurations need to be followed also their effects need to be inspected at quality stages.

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