Testing Solder Joints – How to Do it!

The methods for the evaluation of the solderability test properties, by a visual inspection of wettability and methods, to measure the solder joint strength.

“What is Solderability” Definition & Explanation

Solderability is defined as the ability of a metal surface to be wetted by molten solder.For good solder joint established the electronic component it is recommended that component 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.

1. Wetting;
2. Non-wetting; and
3. Dewetting

Solderability Test Methods and requirements

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.

1. Dip and Look Solderability Test Method

In the dip and look test, the test object is completely immersed in a pot of molten solder. After a defined time it is pulled out and kept for a cool down. After That, the soldering point is visually inspected and the solderability is evaluated by determining the percentage area of wetting.

It is a manual method that also applies to the solderability testing of substrates. For the component or the circuit board to be considered acceptable if no nonwetted dewetted area should be present. Ultimately, It is best to have 100% coverage but 95 % or greater also acceptable. This Method is subjective so generally difficult to estimate the nonwetted /dewetted area.

2. Wetting Balance Solderability Test Method

There is, a second commonly used method for solderability test is the Wetting Balance Test. So, The Wetting Balance Test is conducted to measure the wetting forces between molten solder and the PCB as a function of time. In this solderability test procedure, the lead is dipped in a solder pot, and the time required to reach the maximum wetting force is measured.

Wetting Balance Test Method, is a quantitative solderability test the force acting vertically on the specimen is measured at a function of time. The shorter the time to reach the maximum farce, The more solderable the component.

The Wetting Balance Test Method, does not have established accept/ reject criteria.It is intended for evaluation purposes only.

Visual inspection

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 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 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 test 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.

Test Values

The table below gives only preliminary values for some components, because the method is under developing.

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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