Through-Hole Assembly Process

Through-hole assembly has been one of the foundational elements in electronics manufacturing since the birth of printed circuit boards over 60 years ago. While surface mount technology is rapidly gaining traction in recent decades, through-hole technology remains very relevant in many critical applications where strength, reliability, and high power are not compromised. At PCBX, we are fully aware of the relevance that through-hole assembly still has and the vital role it plays in many applications where robustness and cost-effectiveness are called for.

Through-hole technology includes passing the leads of electronic components through pre-drilled holes on a PCB and soldering them for stable electrical connections. Despite the fact that SMT has dominated most compact, high-density applications, through-hole assembly has remained imperative, especially in industries of aerospace, automotive, and industrial equipment, where boards face mechanical stress and harsh conditions.

Key Steps in Through-Hole Assembly Process

In most instances, there are several key stages involved in through-hole assembly. In addition, they require a degree of accuracy and professionalism, especially when high-quality results are desired with assurance. We will now discuss the primary stages:

Component Insertion

Manual Insertion: Conventionally, components are placed manually by hand using tools. Labor-intensive and slower, it is still applicable for prototyping or low-volume orders where setup flexibility and ease of inspection are valued.

Mechanical/Pneumatic Insertion: Automated insertion machines, with vibratory bowls or complex mechanisms, can insert components with speed and consistency over larger runs of production while minimizing manual handling errors.

Odd Form Insertion: Equipment for atypical components includes those needing lead forming or custom tooling, therefore extending the type of parts that can be used from the norm.

Soldering Techniques

Wave Soldering: Dominant in high-volume production, wave soldering involves passing the entire PCB over a wave of molten solder. This method efficiently solders all joints simultaneously, ensuring repeatability and high throughput.

Selective Soldering: Employed when a board combines through-hole with sensitive SMT components, selective soldering lays molten solder only where required, thereby precluding any damage from a wave process.

Hand Soldering: It is still applicable today for prototypes, repairs, and small batches. Hand soldering is dependent on operator skills but allows more control over each joint, thus is quite suitable for work that requires a lot of precision.

Cleaning and Finishing Steps

After soldering, there may be a cleaning stage following soldering to remove flux residues, which will help the board last longer. The process also includes adhesive curing, conformal coating, and stringent testing, among others, in completing the assembly.

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Design Considerations for Optimal Through-Hole Assembly

Effectively designing PCBs for through-hole assembly involves a specific set of considerations to facilitate manufacturability and durability.

Component and Lead Spacing: The spacing should be enough to avoid any probable defects such as tombstoning or solder bridges. The design should take into consideration the physical footprint and orientation needed for reliable insertion and soldering.

Thermal Relief Pads: These should be employed around heat-sensitive components to control heat transmission, reducing the risk of thermal damage during soldering.

Solderable Surfaces: The engineers should ensure that pads are accessible for full solder coverage, and no obstruction may obstruct the process.

Mixed Technology Strategies: Positioning should be done in such a way that when SMT is integrated, the risk of damage during reflow is minimal. Through-hole components shall be used where strong connection and high mechanical stability are required.

Common Defects in the Through-Hole Assembly

Despite maturity, through-hole assembly can still have defects regarding soldering and component placement. Identification and prevention of these are essential:

Tombstoning: This defect is related to the lifting and standing of the components vertically while soldering. It can be minimized by proper lead spacing and enough clinching after insertion. Solder Skipping: Missing solder fillets are usually due to insufficient heat or contaminated surface conditions. Regulated thermal profiles with clean contacts will help address this issue.

Solder Bridging: This can be avoided by keeping enough spacing between pads and using solder masks to avoid the accidental connection of adjacent leads.

Cold Solder Joints: Insufficient heat or oxidation of surfaces causes dull, weak joints. The solution is in temperature adjustments and verification of the accuracy of the process.

Rework and Repair Strategies

In case defects are present, through-hole assemblies have accommodating repair options:

Manual Soldering: This allows for precise heating to remove and replace faulty components.

Desoldering Tools: Employ vacuum or thermal technology to remove faulty components with minimum damage.

Reclip and Bypass: In cases of more serious faults, component leads can be clipped and bypassed with jumper wires.

Process Control Implementation

Good quality through-hole assembly requires rigorous process control in many aspects:

Solderability Testing: Proper hole wall wetting indicates good soldering and advises process adjustments.

Lead Testing: Clinch height and pull force testing prevents tombstoning.

Thermal Profiling: Thermocouples facilitate monitoring heat distribution and time for soldering, reducing defects.

Automated Optical Inspection (AOI): Provides oversight through verification of placement accuracy and formation of solder joints. It is done by using statistical process control to identify opportunities for continuous improvement.

Mixed Technology Assembly

For a large number of today's electronics, mixing through-hole and SMT components on the same board brings substantial benefits. This hybrid approach leverages the strengths of both technologies:

THT: Better mechanical stability, especially power handling; easy to inspect or repair.

SMT: Designs can be much compact and closely packed with lower assembly costs at quantity levels.

Progressive introduction of SMT on a through-hole infrastructure takes care of technology evolution, avoiding unnecessary loss of capital and maximizing investments. Early implementation or use of SMT could be confined to drop-in replacements for standard Through-Hole components. As SMT capability improves, designs can migrate to higher-density solutions with few through-hole components.

Despite the development of SMT, through-hole technology keeps its niche as a relevant technique in electronics manufacturing. We at PCBX preach its continued integration with SMT for a balanced synthesis of reliability, performance, and efficiency. Through-hole assembly, if executed with precision and backed by strong process controls, continues to provide unrivaled robustness and reliability in application and has become indispensable for modern and legacy electronic designs alike.