The methods for the evaluation of the solderability test properties, by a visual inspection of wettability and methods, to measure the solder joint strength. The solderability test determines whether SnPb, Pb-free, or tin-plated terminations of device packages can be soldered to another surface.
The destructive procedure will be used to determine, using packaging materials and manufacturing processes, whether the component can successfully be soldered during the fabricated assembly
What is Solderability?
Solderability is defined as the ability of a metal surface to be wetted by molten solder. For a good solder joint to establish the electronic component it is recommended that the component is strongly attached to the board substrate.
Good solderability requires wetting (low contact angle) of the substrate by the solder. This applied to components and Circuit boards alike. Suffice it to say that, The surfaces with poor solderability cause soldering defects. It means that the rework costs increase and the reliability of the product is compromised. In this way, components require good solder joints because they are subjected to mechanical and thermal stress during product handling, transportation, and field services.
To describe the solderability for components performance there are three basic mechanisms and functional conditions of molten solder on the base material surface. That we shall describe in detail in turn.
- Non-wetting; and
Solderability Test Methods and requirements
Your PCB tester will be able to determine if the attached components, such as leads and terminations, have the ability to endure the high temperatures involved in soldering.
There are various standardized solderability test procedures that are used by the industry. The two most commonly used tests are the dip and look test and the wetting balance test. Both methods have been regarded as the most versatile for standardized assessments but the dip and look test is more common. The third type of test, The globule test is widely used in Europe. All Solderability tests are accepted by IPC, EIA, and the military services. Let’s get down to the details, IPC-J-STD-001 for solderability test for components and IPC-J-STD-003 for boards are used as industry standards for evaluating solderability.
Dip and Look Method
During the dip and look test, an object is immersed in molten solder for a period of time. We pull it out and let it cool down for a predetermined time. Afterward, the soldering point is visually inspected and the solderability is determined by measuring the percentage area of wetting
Methods of testing substrate solderability can also be carried out manually. In order for the component or circuit board to be considered acceptable, there should be no nonwetted dewetted areas. In the end, it is best to have 100% coverage, but 95 % or more is also acceptable. Due to this method’s subjective nature, it is generally difficult to estimate the nonwetted / dewetted area.
Wetting Balance Analysis
In addition to a Wetting Balance Test, there is a second commonly used solderability test method. In other words, the purpose of the Wetting Balance Test is to measure the wettability of molten solder to PCBs over time. Solderability tests involve dipping a lead into a solder pot and measuring the time it takes to reach maximum wetting force.
This wetting balance test method is a quantitative test of solderability. It measures the force acting along a vertical axis as it varies with time. A component is more solderable when it takes a shorter time to reach its maximum farce. There are no set acceptance/rejection criteria for the Wetting Balance Test Method. This is only to be used for evaluation purposes.
Globule Test Method
This method has become popular in recent years in which a copper wire is lowered horizontally into a molten solder globule on a surface that will not wet the solder (i.e. glass or stainless steel plate). Based on the length of time the copper wire takes to become enclosed in the solder, the wettability of the solder is determined. A qualitative assessment of wettability can be quickly obtained, but quantitative measurements are not always possible.
The Globule test is generally used to measure the wetting time for through-hole components but it can also be used for surface mount devices. This solderability test method is widely used in Europe and defined in IEC-68-2-20. Like the balance wetting test, This Globule test Method also does not have any acceptance or rejection criteria. This method is used for evaluation only.
For good solder joint established the electronic component it is recommended that component strongly attached to the board substrate with solderability test for components below recommended points-
- Component pad and PCB pads both are aligned.
- No excess solder onto the component pad.
- Solder joints are smooth and shiny without void presence.
- PCB pad area completely covered with molten solder.
Inspection for Thermal Damage
Excess heating during soldering may result in different types of failures like component measling and blistering, lifted component, burned or melted insulation or burns on base materials, component creak, or damages.
That can be sorted out by the visual inspection by the naked eye or magnification equipment and do solderability test for components by visual inspection.
During solder joint formation the solder amount must be not exceeding to form a solid solder joint and be concave in the inspection. If the solder amount in excess, the excess solder defect will not be recommended for smooth joint formation.
Solder Joint Strength test
Depending on the component or terminal design the test solderability test procedure may be done by pull, shear, or push off test,
If the component supplier has specified a test method and force value, then this information shall be used as long as the stated force is within the value required for the end product. Fix the test board and apply a force to the component or component lead according to the requirement stated for the component or as indicated below. Apply the specified force gradually at a constant rate.
Solder Joint Strength Test Methods
Pull Test:- Pull test can be used for solderability tests for components. The force must be applied with an angle of 90 +/- 5 degrees to the test board.
Shear Test:- Using the shear test, the force must be applied in such a way that the shear force in the solder joint is parallel to the board. Do not place the pushing tool so it will touch the solder joint.
Push off test:- Push off test can be used for leadless components (bottom only termination) with a few terminals. Apply the force in such a way that it will be equally distributed to the terminals. The test of leadless components with many terminals may be done by soldering only a few terminals and using the shear test method.
The solderability test procedure specified force shall be reached within 5 seconds and maintained constant for the time stated. Alternatively, increase the force gradually until the component or lead has been sheared/peeled off from the test board and measure the maximum obtained force value.
The table below gives only preliminary values for some components because the method is under development.[wptb id=840]
These values are given by some component suppliers in their specifications. However, the values are very low and may not meet the requirements for the end product.
Micro Section Analysis
In addition to the result from the wetting and force test, further analysis of the solder joint may be required if a lower value than required or expected has been obtained from the test.
A rough study of the fractured surface of the tested joint can be performed but is only to be seen as another input to the material graphic analysis.
Fractography of such a complex joining system as a solder joint is often very difficult, but the following is a simplified guideline. A rugged surface indicates.
that the rupture is internally in any of the metals or alloys (e.g. in the solder matrix), while a smooth fractured surface indicates a rupture in an interface (e.g. solder to Ni-barrier). The experience is that a rugged surface gives a higher force value than a smooth surface.
To fully determine the cause for the unsatisfactory strength of the solder joint, micro sectioning of tested and untested joints followed by suitable cinematographic analysis is often the most applicable method.
- Voids or cracks in solder joints.
- Ball grid array (BGAs) ball soldering inspection
- Intermetallic formation and tin whisker growth inspection
- Deep analysis of the amount of solder build-up and joint formations,through-hole filling wetting conditions, and voids in solder joints.
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