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Halogen-free PCB Reliability Test and Failure Analysis

The Printed Circuit Board (PCB or PWB) is the base of electronic products. A PCB integrates and connects various electronic components to form a product assembly.

PCB has a history of several decades. In the recent trend of miniature and light-weight electronic products added with promotion of lead-free processes, the assembling temperature of either reflow soldering or wave soldering gets higher, therefore the potential influence resulting from the additional thermal stress against the tranditional PCB becomes worse, hence product defects attributed to PCBs also worsen when compared to that before the transfer to lead-free processes. Although material suppliers developed high Tg substrate or improved hardening agents to overcome the thermal stress issue, other problems occurred, e.g., reduced Bond Pad Strength, poor hole-drilling quality, and Conductive Anode Filament (CAF) effect. Therefore PCB Reliability Test once more becomes highly focused.

Apart from the issue of lead-free application, international non-government organizations (NGO) actively promote Halogen Free processes. Meeting requirements of lead-free processes and Halogen limitations, We have developed a reliability test addressing properties of PCB materials to assist customers in obtaining certificate of preliminary product reliability tests or product sampling analyses.

Services concerning Halogen-free PCBs

  Temperature Cycling Test and Dynamic Low-Resistance Measurement 

Temperature Cycling Test (TCT) of PCB is the most popular and most important approach. TCT is normally accompanied with a dynamic resistance measurement system, e.g., a Datalogger or and event detector. The main purpose of TCT is to make use of the “CTE (Coefficient of Thermal Expansion) Mismatch” effect between ingredients with different thermal expansion coefficients. Quality risks in the product can be identified through the long-period high-/low-temperature cycling that leads to through-hole cracking and/or product delamination problems. This approach is highly beneficial to multi-layer products and HDI products; it can also identify and improve the strength, plating quality and process stability of the Lamination Bond. Temperature Cycling test can be performed together with non-dynamic tests where a set number of cycles is selected for measuring resistance values.

PCBs used for the temperature cycling test can be of the daisy chain design or of the actual products. The daisy chain design enables decrease of the sample number and provides more thorough observation of the product, which are more helpful in qualifying new suppliers. The TCT test normally complies with the TM650 method 2.6.6, or is based on IEC60068 recommendations. 

Wet/dry Thermal Shock Tests

PCB Thermal Shock Test (TST) aims to verify potential problems owing to sudden exposure to temperature differences and possible issues resulting from frequent temperature alterations, including bodily damage, discoloration, resistance variation, etc.

The main difference between TST and the traditional TCT is that the temperature alteration is gradual in TCT, which aims to manifest structural problems of the product by way of CTE mismatching. In TST, the transit time between different temperature troughs is only 10 seconds, this short interval is insufficient for manifesting CTE mismatch, the test therefore aims to induce material failure of the sample as a result of sudden temperature changes (thermal shocks).

There are two types of TSTs, those with liquid chamber and those with gas chamber; both types comprise double chambers (a cold chamber and a hot one). As a sample is moving from one chamber to another, the reside time includes the transit time, the tested sample must achieve the required temperature (changes) within two minutes. Based on test conditions suggested in IPC-TM-650 method 2.6.7, FR4 materials use condition D and FR5 materials use condition E; total number of cycles depends on IPC specified suggestions with a minimum of 100 cycles.

PCBs used for TST can be of the daisy chain design or of the actual products. The daisy chain design enables decrease of the sample number and provides more thorough observation of the product, which are more helpful in qualifying new suppliers. 

Electrochemical Migration Test 

The Electrochemical Migration (ECM) Test becomes a more important test method after entering lead-free process. Different from the conducting resistance test, the ECM Test measures variation of high-resistances for identifying short-circuit risks between two isolated circuits.

ECM is a phenomenon when a metal migrates to another polarity via ionization, with dendrite deposits formed causing short-circuits, or that ionized metal compound extends within inner layers of the PCB, reaching another conductor to form a connection. The difference between the two conditions is the reaction equation; the location of the migration can be on the face of PCB or within PCB layers. For testing on-face migrations, the test is named the Surface Insulation Resistance (SIR); for testing migrations between the face and an internal layer, or between the internal layers, the test is named Conductive Anode Filament (CAF).

PCBs used for this test is of comb design, actual product PCB can also be tested. Using the comb design PCB is helpful in using lesser number of samples and providing more thorough observation of the product, which are more helpful in qualifying new suppliers.

The results of CAF tests are poorer than that of traditional FR4 or High Tg FR4 materials mainly due to material properties and an adverse influence to the quality of PCB drill or laminate process which further worsens CAF risks. 

Copper Trace Tension Test and solder pad bond strength test 

The Copper Trace Tension Test is a confirmation test after the press assembly of PCB semi-finished product. This test only benefits the semi-finished product; for products with multiple subsequent processes, the benefit is not obvious.

Using the Solder Pad Bonding Strength Test may be of more significance mainly because the test condition is similar to that of the actual product, being capable of simulating the risk of peeling off of solder pads during the maintenance process after the assembly. 

So IPC-9708 has been created, but no acceptance specification is set for this test item; the standard deviation of test data or Cpk is normally used to measure its quality stability. Historical data can also be used for results comparison.

For halogen-free materials, most documentations use the Peeling Test to make observations, bounding strength of these materials is deemed weaker than that of typical FR4 materials. However test results observed from solder pad strength test are not positively related with that of peeling tests. The previous treatments to the Solder Pad Bonding Strength Test as follows; they comprise a simulation of the maintenance operation.

Step 1:Solder wire into lands 

Step 2:Remove (desolder) wire from lands 

Step 3:Resolder wire into lands 

Step 4:Remove (desolder) wire from lands 

Step 5:Resolder wire into lands

 Simulation of Heat Resistance 

In general the PCB heat resistance test is simulated by way of 288°C solder floating. Since the current products are mainly assembled with SMT components, therefore a reflow simulation will be more effective; however this approach has not yet been included as a standard method. IPC is currently being discussed by the MSD Council, the moisture absorption level of PCB is expected to be of Level 3.

Daisy Chain can be adopted to monitor resistance measurement of this test; after each test, a measurement shall be made to verify the number of failure occurrence. This approach can also be used for process quality monitoring, agreeable results are seen in quality monitoring of press stations and plating process.

 Dynamic thermal oil test 

The Dynamic Thermal Oil Test is not truly a reliability test. Monitoring conductive resistance via the daisy chain, actual position and conditions of the failure can be verified.

The following figure shows conditions of a Thermal Oil Test. The method provides prompt identification of early failure spot of the process, which can be used as a confirmation of process quality during the product development. After quality improvement, the same test can be used again. 

CTE and Tg Measurement 

Coefficient of Thermal Expansion (CTE) and Glass Transition Temperature (Tg) are basic test items of Halogen-free substrates. Due to large amount of metal hydrates are used as fillers in Halogen-free materials, the pressing condition in the process must be properly adjusted. The press quality is closely related to the bonding strength of the product, delamination or popcorn effects are frequently seen in reflow or wave soldering processes due to thermal shocks, which further affect the connection of vias in the vicinity. This measurement can be carried out with TMA or TGA in general. However, Halogen-free materials are not as mature as the conventional FR4 materials; further confirmation of the theoretical Tg, delta Tg and CTE are required prior to the tests for assisting the determinations.

 Bending Test 

Owing to the use of large amount non-organic metal hydrate fillers, Halogen-free PCBs tend to becoming hard and fragile. The main purpose of the bending test is to observe the risk taking quality of vias and the board material under bending conditions. Bending tests can be categorized into destructive and non-destructive ones that shall be selected based on different requirements or assembly conditions.

For portable products, the most frequently used bending tests are cyclic bending, for the purpose of simulating material fatigue when the product is subject to a continuous bending. In addition, the PCB can be tested with components assembled, for observing solder joints affected by the strain resulting from the external force exerting on the PCB during the test. Typical problems like pad peeling and pad cratering can be revealed through this test.

In addition, by way of single and continuous bending, Halogen-free board appears to be of high hardness; via daisy chain monitoring, the number of defects found in Halogen-free boards is clearly higher than that in traditional high Tg FR4 boards under the same bending stress. 

Mechanical Shock Test 

Although Mechanical Shock Tests are also performed against PCB raw materials, yet the stress distribution on blank boards is not quite sufficient for revealing structural problems of the product, therefore Mechanical Shock Tests are mainly for the assembled products. Under the impact in a high G value, the deformation in the PCBA, results in the creep of solder joint between the PCB and the component, frequently causes a bond failure. This problem becomes a harsh challenge to the bonding strength in Halogen-free boards owing to their increased hardness and decreased ductility; especially when the component pitch gets smaller and smaller in the future days, the risks deserve further studying.

 Failure Analysis 

As described previously, many failures have been eradicated thanks to the maturing of PCB development processes over decades. With the introduction of the lead-free process and halogen-free materials, however, the industry is forced to re-examine potential issues and the associated phenomena due to the change of materials.

We currently possesses PCB reliability test in accordance with IPC-TM-650, can be distinguished into chemical tests, mechanical tests, environmental/reliability tests and SMT assembly simulations, for assisting customers to perform serial tests together with consultation on relevant technologies and failure analysis services.

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