The methods for the evaluation of the solderability test properties, by a visual inspection of wettability and methods to measure the solder joint strength.
To describe the solderability test for components performance of any there are below three functional conditions of molten solder on base material surface
- Wetting;
- Non-wetting; and
- Dewetting
Visual inspection
For good solder joint established the 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 naked eye or magnification equipment and do solderability test for components by visual inspection..
Joints Contours
During solder joint formation the solder amount must be not exceeding to form a solid solder joint and be concave in inspection.
If the solder amount is excess, the excess solder defect will not be recommended for smooth joint formation.
Solder Wettability Angle Test
A visual inspection of the wettability is done as a first analysis of solderability test for components.Measure the wetting angle on the vertical part of component terminals with a magnifier of 10x to 40x. Angles of 10 degrees or less indicates a very good wettability.
However, the design or the dimension of the terminal may cause a limitation of the wetting length and thereby also the wetting angle. To have an acceptable wettability, the experiences is that the angle should be less than 55 degrees if no limitation of the wetting length exist.
As an alternative the wetting balance test according to 105 82-103 may be used for this purpose.
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 test for components . The force must be applied with an angel of 90 +/- 5 degrees to the test board.
Shear Test :- Using 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 away 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 second 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.
Test Values
The table below gives only preliminary values for some components, because the method is under developing.
Component Packages | Test method | No. of Solder joints | Min time (s) at constant force | Minimum Force (N) |
0805 | Shear | 2 | 10 | 1 |
0603 | Shear | 2 | 10 | 0.75 |
0402 | Shear | 2 | 10 | 0.50 |
0201 | Shear | 2 | 10 | 0.25 |
SOT 23 | Pull | 1 | -- | 20 |
SO 16 etc. | Pull | 1 | -- | 20 |
Leadless ceramic | Push off | 4 | 10 | 5 |
These values are given by some component suppliers in their specification. 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 materialographic 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 experiences 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 amount of solder build-up and joint formations,through-hole filling wetting conditions and voids in solder joints.
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