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New Test for Pad Cratering
Time-temperature profile, where T1 is the preheat region, selected by defining the temperature to reach and the time to achieve this heating period; T1 to T2 is the soak period and the soak time can be selected; T2 to T3 is the rate of rise; T3 to T4 is the reflow period, the reflow time period can be selected; T4 to T5 is the cooling period; T6 is the temperature where the test will be executed.
By Kamran Iqbal, B.Eng (Hons), M.S.c, Senior Applications Engineer, Nordson DAGE
A common failure mode in lead-free assemblies is pad cratering, where the contact pad on a printed circuit board or package substrate lifts away from the surface. There are many factors that cause this failure mode to be more prevalent in lead-free assemblies compared to tin-lead assemblies, these include:
Lead-free solders are mechanically stiffer than tin-lead solders, hence more strain is transferred to the assembly.
Lead-free solder requires higher reflow temperatures and cooling rates when compared to tin-lead. This can lead to increased strains on the assembly.
Mechanical testing such as bend and shock are common practices on surface mount assemblies and printed circuit boards in order to verify product design, ensure quality and ultimately to ensure product longevity. The mechanical stresses applied evaluate the susceptibility of a product to failure.
Although these mechanical test methods test the entire assembly, it can be difficult to differentiate the types of failure modes occurring during the test. Common factors which contribute to different failure modes are:
It is therefore important to isolate the failure modes to identify any weaknesses in the assembly. This is an important part of development and production, as it ensures correct design of product, manufacturing process control, and quality of the final product.
Over the past several years, pad cratering has become a significant problem in lead-free assemblies having been largely attributed to issues with the resins used to manufacture printed circuit boards. To help the industry detect and eliminate this defect, the IPC has recently published IPC-9708 test standard for the detection of pad cratering for surface mount components and printed circuit board assemblies — "Test Methods for Characterization of PCB Pad Cratering," IPC, March 2011,
. This new industry standard provides three standardized test methodologies enabling product developers to determine the best circuit board material for a given application.
Prior to widespread lead-free conversion, these types of failures were relatively rare and therefore did not receive much attention. As lead-free production volumes increased, the pad cratering phenomena came to the forefront with occasional catastrophic failures being discovered in some types of products. Of equal concern is that these pad cratering defects can seldom be detected with traditional testing.
The IPC-9708 standard provides test methods to evaluate the susceptibility of printed board materials and designs relative to cohesive dielectric failures that can occur underneath surface mount attachment pads. The test methods, which include cold ball pull and hot bump pull, ball shear and hot pin pull, can be used to rank the order and compare different printed board materials and design parameters. The standard allows individuals to select alternative board materials, build test coupons and run tests on these coupons in accordance with the IPC-9708 standard to make a material selection decision before building production volumes.
Hot Bump Pull Test
The latest pad cratering standard, IPC-9708, defines hot bump pull as a method to evaluate the susceptibility of printed board assembly materials and designs to cohesive dielectric failure underneath surface mount attachment pads. This method can therefore be used to rank the order and compare different materials and process parameters.
The new Nordson DAGE 4000Plus hot bump pull system is a totally industry unique solution, conforming to IPC-9708 as well as JEITA ET-7407A standards. It is fully integrated into a single load cartridge on a standard 4000Plus bond testing system. Heating, cooling stages and pin clamping mechanisms are integrated into a single load cartridge. The Paragon
bond tester software provides a user-friendly interface with time-temperature profiles to setup a test.
The new cartridge design allows simple straight test pins to be used, allowing maximum force to be transferred as well as providing a low cost of consumables for testing purposes. The new straight pin design provides successful and consistent testing. It is important to pull the pin vertically, directly by the apparatus without imposing any bending moments.
Hot Bump Protocol
The test procedure consists of two parts: set up of the test parameters and positioning of the pin over a solder bump. First a temperature profile is created which allows a user to input time and temperature criteria for reflow and test conditions. This simple interface only requires the desired temperature and the time to reach that temperature. The heating and cooling rates as well as test execution are then automatically handled by the hardware and software.
A temperature/time profile consists of these stages:
Rate of rise.
Performing the Test
A straight pin is vertically inserted into the cartridge, where it is held in place. The pin is then lowered onto the solder bump via motorized horizontal and vertical stages. The pin is set so that it is touching the solder bump. The test is then executed, where the pin temperature ramps up according to the defined temperature profile created. At reflow point the pin drops down to a desired level, ensuring a good solder joint. The clamping mechanism then engages and clamps the pin so it is ready to be pulled. The cooling is handled internally by pulsing compressed air past the pin and onto the test sample, cooling at the defined temperature profile. Once the test temperature is reached, the test is automatically executed, recording the force-time, force-distance and energy values.
Hot bump pull test results as shown by bond tester software.
In addition, the 4000Plus system enables an array of destructive and non-destructive testing that can be performed including simulating tensile and fatigue stresses on a solder ball, pad or substrate as well as destructive tensile pull, non-destructive tensile pull and fatigue cyclical tensile pull testing.
Failure Mode Inspection
Conducting a hot bump pull test and producing a failure mode is one part of the test. The other is to capture the visual data and be able to store it in a convenient manner, and not slowing down the testing operation. The 4000Plus has the capability to take detailed images in between tests. This is achieved via the optional built-in image capture camera. The camera can be set up to take a high resolution image of the area under test and store it for analysis or reporting.
The test operator can also allocate appropriate failure modes with a user-friendly graphical user interface, which is important for failure analysis. Reports can be generated where force, data, failure mode and image can be presented.
Board level manufacturers who utilize the IPC-9708 standard in conjunction with the hot bump pull test methodology will see substantial cost reductions by reducing the burden of verification and qualification of alternative board materials. Material testing is costly and time consuming and standardized test methodologies significantly quantify which materials performs better in end-user environments. As is the case with many standards, widespread adoption will bring significant benefits to the industry as a whole.
Contact: Nordson DAGE, 48065 Fremont Blvd., Fremont, CA 94538-6541
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