Home Preventing Electrochemical Migration in HAST

Preventing Electrochemical Migration in HAST

Home Preventing Electrochemical Migration in HAST

Preventing Electrochemical Migration in HAST

by ruby

Issued Date:2022/7/12 hast-ecm
Issued By:iST

Why does ECM take place in HAST?
Is it a flux issue or the test environment issue?
How can we prevent ECM in HAST?

In reliability testing, there is a test item called the Highly Accelerated Temperature and Humidity Stress Test (HAST). It aims to test the resistance against temperature and humidity of IC packaging to ensure product reliability.

In this test, we need to connect an external power supply unit (PSU) to a pressure cooker to supply DC voltage and then it connects to the HAST board and IC socket containing the IC being tested.

Although this test looks simple, based our reliability test experience for over 20 years, there are some difficulties that customers must overcome. Particularly, when chip application is increasingly complex and chip precision keeps improving, and ball grid array (BGA) and chip scale package (CSP) have become the commonly used IC packaging methods and solder ball pitches are smaller, electrochemical migration (ECM) takes place very easily to cause short circuit in HAST. When this happens, you must ask: “Which step in the process will make this happen? Is this a control problem in the experiment environment?”

The iST Classroom of this issue will review this issue in depth from outside in and share with you how to prevent ECM in a HAST (HAST-ECM).

hast-ecm

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What is electrochemical migration (ECM)?

Due to the effect of the electric field, a conductive channel will form between the anode-charged and cathode-charged metal icons (Figure 1) to produce Electrolytic Corrosion. As ions are released like tree branches and combine with one another from different areas (Figure 2), this causes a short circuit. EMC commonly takes place on PCBs (Figure 3).

HAST-ECM

Figure 1: Principle of ECM

HAST-ECM

Figure 2: Brach-shaped Metals Released by ECM.

PCB ECM

Figure 3: ECM Takes Place in a HAST (often found on PCBs)

  • 1. Main Cause of ECM

    Electrolyte layer formation is the main cause of ECM. Free ions will be produced from electrolyte layer, which increases electric conductivity. Humidity, temperature, sweat, contaminants in the environment, flux chemicals, PCB materials, surface coarseness, and others are the common factors accelerating electrolyte layer formation. Hence, preventing electrolyte layer formation is crucial.

  • 2. Key to the Chance of ECM: Flux Cleanliness

    After chips are assembled into BGA packages, flux is used in balling to ensure the smooth connection of two different metals or alloys. Our observations have found that if flux is not cleaned after balling, in addition to blocking the underfill flow (Figure 4) to prevent underfill from filling up the chip bottom, flux residue will form many bubbles (further reading: Eliminating Underfill Voids With the Vacuum Pressure Oven) and even increase the chances of ECM.

    HAST-ECM

    Figure 4: Without cleaning, flux residue will block the underfill flow.

    At iST, we have developed two designs of experiments (DOEs) using the common HAST requirements: temperature at 130°C and humidity at 85%RH, and common clean-free flux is used. In DOE1, flux cleaning is not performed. In DOE2, flux cleaning is performed. The results show that ECM (Figure 5 (left)) takes place in DOE1 (flux cleaning not done), while only mild ECM (Figure 5 (right)) is noted in DOE2 (flux cleaning done).

    ECM Experiments With Clean-Free Flux

    DOE1:
    Temperature: 130°C
    Humidity: 85%RH
    Flux Cleaning: Not Done

    DOE2:
    Temperature: 130°C
    Humidity: 85%RH
    Flux Cleaning: Done

    Figure 5 (left)

    Figure 5 (right)

    In these experiments, we found that flux residue cannot be exterminated even if flux cleaning is performed. Hence, whether wet flux cleaning (Want to know more about wet flux cleaning? Write to web_cre@istgroup.com or marketing_tw@istgroup.com for more information) affects ECM intensity in a high-temperature and high-humidity experiment process should not be overlooked.

    Certainly, the flux clean methods, processes, time control, and solvent types should be adjusted according to the size and quantity of samples, the flux types, and others for effective cleaning.

  • 3. Environment and Maneuver: Factors Affecting EMC

    HAST is performed in a pressure cooker with complex wiring prone to dirt deposition. Hence, periodic cleaning and maintenance and regular service according to the manufacturer’s instructions are required to maintain the interior cleanliness of the machine.

    Additionally, operators must wear gloves at all times during the experiment to prevent contamination by sodium ions, potassium ions, calcium ions, and others in sweat.

  • 4. Timely ECM Detection in HAST

    Although the above precautionary actions can effectively help reduce the risk of EMC significantly, how can one detect ECM or abnormal power consumption in the middle of an experiment when mistakes are made in other steps of the experiment?

    At iST, the power management system and power supply module (Figure 6 (left)) are used together with a custom circuit design to improve automation performance and implement power supply monitoring at the same time to ensure that each HAST board is under independent control. Additionally, the dynamic verification process record (Figure 6 (right)) in the experiment can ensure stability throughout the experiment and help detect anomalies early to prevent serious damage to the product or HAST board from disrupting subsequent failure analysis.

    HAST-ECM

    Figure 6 (left): Power management system and power supply module.
    Figure 6 (right): Power supply monitoring and dynamic verification process record.

In this article, we share the experience in reliability test with you, a long-time supporter of iST. As Asia’s largest third-party verification and analysis lab, with sufficient field experience, iST offers the most complete and perfect equipment distribution and technical consultation services, ranging from the planning of initial experiment conditions to detailed environments; the design of reliability tests for hardware assessment, circuit and layout design; and the test for hardware production and manufacturing, to subsequent reliability test implementation and anomaly analysis and elimination. Our services can help you quickly and accurately run various reliability test requirements and provide you with consultation services for various products, such as consumer electronics and automotive electronics.

If you have doubts on the Reliability test, you are welcome to contact Ms. Lynn Peng at +886-3-579-9909 ext. 6902 or write to us at web_cre@istgroup.com or marketing_tw@istgroup.com

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