The most important factor in determining solder quality is its ability to flow from molten to solid. The easiest way to test a solder’s ability to do this is by performing a solderability test.
A solderability test measures how well a joint will withstand heat and mechanical stresses. The Solderability Test is a test that is used to determine if a solder joint is good or not.
This test ensures that the solder will bond to the device and hold up to the vibration and other stresses that the device will be subjected to. The destructive procedure will determine whether the component can successfully be soldered during the fabricated assembly using packaging materials and manufacturing processes.
I bet you didn’t know there are many ways to measure solderability. There are even specialized tools for it. This article will get you up to speed on all the different approaches.
What is Solderability?
Solderability measures how well a solder joint holds in a circuit board. It is typically expressed as a percentage; the higher the percentage, the better. When a circuit board is manufactured, they are typically made with a surface finish that provides good solderability.
Solders are metal alloys that melt at a lower temperature than the metals used to join. They usually contain lead, tin, silver, indium, zinc, copper, bismuth, antimony, and arsenic. Solderability is the quality of a soldered joint’s strength and the ease with which it can be made mechanically.
To describe the solderability for components performance, there are three primary mechanisms and functional conditions of molten solder on the base material surface that we shall describe in detail.
- Non-wetting; and
- Wetting;
- Dewetting
Solderability Test Methods and requirements
Solder is typically applied to a joint by a process called soldering. There are several solderability test methods that can be used for this process.
The industry uses various standardized solderability test procedures. The two most commonly used tests are the dip and look test and the wetting balance test. Both methods are the most versatile for standardized assessments, but the dip and look test is more common.
Your PCB tester will determine if the attached components, such as leads and terminations, can endure the high temperatures in soldering.
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
Understanding the solderability of a PCB is a very important process to ensure a high-quality product. One method for understanding solderability is the “Solderability Test Dip and Look”.
This article will discuss the Solderability Dip and Look method, a great way to ensure that your solder joints are done correctly.
An important part of soldering is the “dip and look” test. This test is used to determine the quality of a solder joint and to see if there are any problems. This test is performed by dipping the component into molten solder at a temperature above the melting point of the solder alloy, and then we pull it out and let it cool for a predetermined amount of time.
After the test, the solder is visually inspected, and the quality of the solder is determined by measuring the wet area percentage.
Methods of testing substrate soldering can also be carried out by manual Inspection. There should not be any non-wetted areas on the component or circuit board that are not accepted. The goal is to have 100% coverage. However, 95% or more is also acceptable, as long as you can cover the entire area.
The dip and look solderability testing method is not recommended for use with leaded or surface mount components because it does not provide an accurate indication of the amount of solder required to attach such components. This method’s subjective and imprecise nature makes estimating the non-wetted / de-wetted area very difficult.
Wetting Balance Analysis
For those who work in electronics manufacturing, there is a process called the Wetting Balance Analysis Solderability test. Wetting Balance Analysis Solderability is a test used to measure a solder’s surface tension.
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 wettability assessment 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 does not have any acceptance or rejection criteria. This method is used for evaluation only.
Visual inspection
For a good solder joint establish the electronic component, it is recommended that the component firmly attached to the board substrate with a solderability test for components below recommended points-
- The component pad and PCB pads are both 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 failures like component measling and blistering, lifted components, burned or melted insulation or burns on base materials, component creak, or damages.
That can be sorted by visual inspection by the naked eye or magnification equipment and solderability test for components by visual inspection.
Joints Contours
The solder amount must not be exceeded during solder joint formation to form a solid solder joint and be concave in the inspection. If the solder amount is 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, 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 tests can be used for solderability tests for components. The force must be applied to the test board with an angle of 90 +/- 5 degrees.
Shear Test:- Using the shear test, the force must be applied so that the shear force in the solder joint is parallel to the board. Please do not place the pushing tool to 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 so that it will be equally distributed to the terminals. The test of leadless features with many terminals may be done by soldering only a few 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.
Test Values
The table below gives preliminary values for some components because the method is under development.
[wptb id=840]Some component suppliers give these values in their specifications. However, the values are shallow and may not meet the requirements for the end product.
Some component suppliers give these values in their specifications. However, the values are shallow and may not meet the requirements for the end product.
These values are based on “in-house” practical experiences and reflect the value achieved on an approved solder joint
Micro Section Analysis
In addition to the wetting and force test result, further solder joint analysis may be required if a lower value than needed or expected has been obtained from the trial.
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, graphical 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 insufficient 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 solder build-up and joint formations,through-hole filling wetting conditions, and voids in solder joints.
