Common Failures in TO Packages and How to Prevent Them

In high-reliability electronics, TO (Transistor Outline) packages play a crucial role. Yet, even these trusted workhorses can fail—often with serious consequences.

This article covers the main failure types in TO packages, their causes, and how to prevent them. Understanding issues like thermal stress and seal failure is key for designers, engineers, and quality teams working with semiconductors, optoelectronics, and power devices.

What Are TO Packages?

TO packages are a family of standardized metal-can enclosures widely used to house transistors, diodes, photodiodes, laser diodes, and other discrete semiconductor components. Common types include TO-18, TO-39, TO-46, TO-92, TO-220, and TO-247, among others.

These packages are valued for:

• Excellent thermal performance

• Robust mechanical structure

• Hermetic sealing for harsh environments

• Easy heat sinking and mounting

Despite their strengths, TO packages can fail under certain conditions—often silently and without external clues.

Hermetic Seal Failures

Cause: Hermetic seals protect the internal semiconductor from moisture, dust, and contaminants. However, poorly executed welding, glass-to-metal seal cracks, or corrosion can compromise this barrier.

Symptoms: Fogging, increased leakage current, corrosion on leads, or outright device failure.

Prevention:

• Use helium leak detection (e.g., MIL-STD-883 Method 1014)

• Implement robust quality control during seam welding or glass sealing

• Select compatible materials for thermal expansion matching

• Avoid mechanical stress during handling and assembly

Lead Fatigue and Cracking

Cause: Mechanical stress from vibration, bending, or improper insertion can cause cracking at the base of leads or inside the package.

Symptoms: Intermittent connections, increased resistance, or open circuits.

Prevention:

• Adhere to insertion and bending radius guidelines

• Use strain relief in mounting and PCB layout

• Employ compliant lead designs or use flexible interconnects in high-stress environments

Die Attach Failures

Cause: The semiconductor die is bonded to the package’s header, often with solder or conductive epoxy. Thermal cycling, poor bonding materials, or voids can cause delamination or cracking.

Symptoms: Thermal resistance increase, erratic behavior under temperature variation, or catastrophic failure.

Prevention:

• Perform X-ray or acoustic microscopy to inspect voids

• Choose appropriate die attach materials with matched coefficients of thermal expansion (CTE)

• Use controlled reflow profiles for soldering

Internal Wire Bond Failures

Cause: Wire bonds connect the die to the external leads. Vibration, corrosion, or bond lift-off due to poor adhesion can disrupt this critical connection.

Symptoms: Intermittent operation, signal distortion, or complete failure.

Prevention:

• Use gold or aluminum wires appropriate to the application

• Optimize ultrasonic bonding parameters

• Conduct bond pull tests as part of quality assurance

• Protect internal wires with gels or encapsulants where vibration is a concern

Thermal Overstress

Cause: TO packages are often used in power applications. Inadequate heat sinking or sudden power surges can result in die overheating or thermal fatigue.

Symptoms: Burnt die, decreased device lifespan, degraded electrical performance.

Prevention:

• Design proper heat sinks and thermal vias

• Perform thermal simulations under real use cases

• Monitor junction temperatures using embedded sensors

• Follow datasheet maximum ratings strictly

Moisture Ingress and Corrosion

Cause: While TO packages are often hermetic, flaws in sealing or long-term exposure to moisture can still allow ingress.

Symptoms: Corrosion on leads or die, erratic current leakage, or bond degradation.

Prevention:

• Perform rigorous preconditioning and soak testing

• Store devices in moisture barrier bags (MBBs) with desiccants

• Use conformal coatings in particularly humid or marine environments

Package Cracking

Cause: Stress from PCB mounting, rapid thermal expansion, or impacts can result in cracks in the header or seal area.

Symptoms: Package deformation, broken leads, seal failure.

Prevention:

• Use mounting hardware that doesn’t over-tighten or misalign

• Avoid rapid thermal cycling

• Select packages with suitable mechanical compliance for the application

Electrostatic Discharge (ESD) Damage

Cause: TO-packaged devices, especially MOSFETs or laser diodes, can be very sensitive to static electricity.

Symptoms: Catastrophic or latent failure; often indistinguishable without testing.

Prevention:

• Use grounded wrist straps, mats, and ionizers in handling areas

• Store and ship in ESD-protective containers

• Apply on-die protection circuitry where possible

Failure Rate Statistics in TO Packages

Best Practices for Engineers and Manufacturers

• Design for Reliability: Choose TO packages that match your application’s electrical, thermal, and mechanical demands.

• Collaborate with Vendors: Ensure your supplier follows rigorous MIL-STD or JEDEC standards.

• Test Early and Often: Integrate accelerated life testing, burn-in, and visual inspection in your quality workflow.

• Invest in Failure Analysis: When failures occur, root cause analysis (e.g., SEM, FTIR, cross-sectioning) informs future prevention strategies.

• Document and Train: Establish handling and mounting SOPs for all stakeholders involved.