The Printed Circuit Board (or Printed Wiring Board) is the critical and unique component of electronics, since PCB is basement and various components are assembled and connected on it, then a system or product could functionalize.

PCB has been developed for decades. In 2006 the implement of Lead Free Process in the assembly of electronic products make the assembly temperature of both Reflow and Wave Soldering to increase, as well as the increase of thermal stress. The potential impact of traditional PCB is more serious, and the root cause to make system product failure (Defect) is getting worse than Pb process before; although many material suppliers have developed High Tg based material and improved filter material to overcome the thermal stress problem, many unpredictable issue occur, such as reduced Bond Pad Strength, poor drilling quality, and Conductive Anode Filament (CAF) increase, etc. Therefore, the reliability validation of PCB is important and must be considered seriously.

In addition to the problems arising after the introduction of the lead-free process, various international organizations are also actively promoting halogen-free products in response to environmental protection. Under the dual requirements, PCB manufacturers must meet the lead-free process specifications and also have to satisfy the halogen-free requirements. In view of this situation, DEKRA iST, as a partner with clients, develops a turnkey solution of reliability verification for PCB product (include material characteristics), and provide clients not only validation during design phase, but also monitor production quality by sampling.


 Total Solution for Halogen-fee PCB   
  • Temperature Cycling Test and Dynamic Low-Resistance Measurement
Temperature Cycling Test (TCT) is the most popular and important approach for PCB. TCT is normally accompanied with a dynamic resistance measurement system, e.g., Datalogger or event detector. “CTE (Coefficient of Thermal Expansion) Mismatch” effect is the main purpose of TCT between different materials while integration. Quality risk of the product can be identified through the long-period high-/low-temperature cycling and normally leads to through-hole cracking and/or product delamination problems. This testing method is highly beneficial to multi-layer products and HDI products, and it can identify and improve the strength, plating quality and process stability of the Lamination Bond as well. TCT can be performed with non-dynamic tests and accompany with measuring resistance values after assigned cycles. PCB for TCT can be adopted the daisy chain design or of the actual products. The daisy chain design enables decrease of the sample size and provides more complete observation of the product, which are more helpful in qualifying new suppliers. TCT condition is normally referring to IPC-TM-650 method 2.6.6 or IEC60068.
 
<IPC-TM-650 2.6.6 conditions>
 
  • Liquid/Air Thermal Shock Test
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 device body damaged, discoloration, resistance variation, etc.

The difference between TST and traditional TCT is that 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 temperatures is only 10 seconds, and this short interval is insufficient for manifesting CTE mismatch. The purpose of TST is inducing material failure of the sample as a result of rapid temperature changes.

There are two types of TST chamber: liquid chamber and air chamber; and both types comprise double chambers (cold zone and hot zone). As a sample is moving from one chamber to another, the reside time includes the transit time and sample arrival must achieve the required temperature within two minutes. According to 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.
 
<IPC-TM-650 2.6.7 conditions>
 
  • 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 surface of PCB or within PCB layers. For testing surface migrations, the test is Surface Insulation Resistance (SIR); for testing migrations between the surface and an internal layer, or between the internal layers, the test is Conductive Anode Filament (CAF).

Comb design PCB or real product both can be executed SIR/CAF tests, and using the comb design PCB is helpful in using less samples and providing more complete observation of the product, which are more helpful in qualifying new suppliers.
The CAF results of Halogen-free PCB is poorer than traditional FR4 or High Tg FR4 materials, and the main reason is material properties and an adverse influence to the quality of PCB drill or laminate process which further raises CAF risks. 
 
<Real-time monitoring resistance changes>
 
  • Copper Trace Tension Test and solder pad bond strength test 
The Copper Trace Tension Test is a confirmation behavior after the press assembly of PCB semi-product. This test only benefits the semi-product, but not for final product after multiple subsequent processes.

Using the Solder Pad Bonding Strength Test is more significance mainly because the test condition is similar to the real product, being capable of simulating the risk of peeling off of solder pads during the maintenance process after the assembly.
<Pad cratering phenomenon of Halogen-free PCB>

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.

 
<IPC-9708 three methods for pad cratering>
 
  • Simulation of Heat Resistance 
In general the PCB heat resistance test is simulated by 288°C solder floating. However, since the current process is mainly assembled with SMT components, a reflow simulation will be more effective. This approach has been included as a standard method, IPC-TM-650.
 
<IPC-TM-650 2.6.27 reflow set-up condition>
  • 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. 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.
<Coefficient of Thermal Expansion example>
 
  • Bending Test
Since the volume non-organic metal hydrate fillers use in the material, Halogen-free PCB becomes hard and fragile. The main purpose of the bending test is to observe the damage risk of vias and laminate material under bending conditions. Bending tests can be categorized into “test to fail” and “test to pass” 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, with single bend and continuously stress by different displacement, Halogen-free board appears the higher hardness than FR-4, and 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.

 
<Comparation of PN FR4 & HF-A testing>
 
  • 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.
 
  • Faiure Analysis
As described previously, many failures have been eradicated thanks to the maturing of PCB development processes over decades. With the implement of 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.
 
<CAF, Conductive Anode Filament, phenomenon>

DEKRA iST currently executes PCB reliability test in accordance with IPC-TM-650, including chemical tests, mechanical tests, environmental/reliability tests and SMT assembly simulations. DEKRA iST not only provides comprehensive validation to clients, but also offers the expertise consultant of relevant technology and failure analysis.
 

 

To make all your PROBLEMS SOLVED, we provide professional consultant and service.

For more information or service, please feel free to email to 📧 sos@dekra-ist.com


 
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