Solderability Testing

A solderability test determines whether molten solder can wet component leads or terminations. Six Sigma Microelectronics performs solderability testing on electronic components to IPC J-STD-002, MIL-STD-883 Method 2003, MIL-STD-750 Method 2026, and MIL-STD-202 Method 208, using the dip and look method (for leaded and leadless components) and surface mount simulation (for BGAs, CGAs, and other surface mount components where dip and look is not appropriate). Other solderability test methods include resistance to metallization dissolution and the wetting balance test, but the industry rarely uses them anymore. Solderability testing is considered destructive; tested components must not be installed in final assemblies. For forty years, Six Sigma has performed this test for commercial, defense, and space programs where one unreliable solder joint is not an acceptable outcome.

WHAT IS A SOLDERABILITY TEST?

Solderability is a metal surface's ability to be wetted by molten solder. Solderability involves two steps: wettability and spreadability. For good solderability, molten solder must wet and flow across the metal surface and spread well to form a continuous, adherent film, rather than bead up and pull away.

A solderability test determines whether a component is solderable, or whether it will remain solderable after long-term storage. Solderability testing is a destructive test — tested electronic components must not be installed in final assemblies.

The solderability testing process flow is preconditioning, then the solderability test itself, then inspection against the acceptance criteria in the applicable industry standards.

PRECONDITIONING: STEAM AGING OR DRY BAKE

Two types of sample preconditioning exist before solderability testing: steam aging and dry bake. Both are accelerated aging steps designed to simulate the conditions components experience during storage or shipping. Steam aging exposes test samples to saturated water vapor to simulate the accelerated temperature and humidity a component experiences, while dry bake exposes test samples to elevated temperatures to simulate the surface oxidation that develops during long-term storage or shipping.

The applicable standard specifies which preconditioning category applies to which component finish. Without steam conditioning or dry bake, a solderability test only establishes that the component was solderable on the day it was tested — not that it will remain solderable when it reaches the assembly line, especially after long-term storage.

Preconditioning categories performed by Six Sigma

SOLDERABILITY TEST METHODS

In general there are four solderability test methods, depending on the component type. All tests must be performed using a mechanical or electromechanical dipping device — no hand dipping is allowed.



Dip and Look Test

The dip and look test is the basic solderability test for leaded and leadless components. This test cannot be used for Ball Grid Array (BGA) or Column Grid Array (CGA) components. In this test, samples are preconditioned, then dipped in an activated rosin flux, and then immersed in a static solder bath using a dipping mechanism.

Every test variable in the procedure must be controlled: flux type and flux activity, solder pot temperature and composition, flux and solder dip dwell times, and the immersion and withdrawal rates of the dipping mechanism. Any deviation from these parameters could result in a false pass or a false failure.

Surface Mount Simulation Test

The surface mount simulation test evaluates actual surface-mount component performance in a reflow process that the dip-and-look method cannot assess. This is the best method for performing solderability testing of BGAs and CGAs, and it may also be used on all surface mount components. After preconditioning, solder paste is screen-printed onto a ceramic plate, the surface mount component is placed onto the paste, and a specified reflow profile is performed. For CGAs, metallized test substrates with pads are used instead. After reflow, CGA samples are cut in two to examine both solder joints.

The dip and look test cannot be used on BGAs or CGAs, because the solder balls or solder columns would dissolve into the bath if they were dipped. The critical surface mount simulation test parameters are the type of solder paste, screen-print stencil thickness, component placement, and thermal profile.

Resistance to Dissolution of Metallization Test

The resistance to dissolution of metallization test verifies that metallized terminations will remain intact throughout the assembly soldering process. It reveals a susceptibility to loss of solderability due to either dissolution of metallization over unsolderable base material, indicated by loss of wetting, or accumulation of impurities from the base metal, indicated by dewetting. The procedure is similar to the dip and look test but uses a longer dwell time — typically 30 seconds rather than 5 — and a higher solder dip temperature of 260°C rather than 245°C. The flux type is ROL1 for both Sn-Pb and lead-free testing.

Wetting Balance Test

Wetting balance analysis is a quantitative solderability test method. The instrument immerses a test specimen in a molten solder bath and records the vertical wetting force on the specimen over time. The resulting wetting curve provides numerical time-to-wet and maximum wetting force values, whereas the dip-and-look method returns only a visual pass or fail.

Because the output is measured rather than observed, wetting balance analysis is used when the objective is to compare one surface finish against another, one supplier against another, or one date code against another, rather than make a straightforward accept-or-reject decision.

Six Sigma Microelectronics does not perform wetting balance analysis. We describe the method here because it is defined in the standards and is occasionally specified on customer drawings.

INSPECTION

After solderability testing, inspect test samples per the applicable standard at 30x magnification for fine-pitch components and 10x for all other components. If defects are found, increase magnification to 70x for fine-pitch components and 30x for all other components for referee verification.

The most common defects found after solderability testing are non-wetting, dewetting, and pinholes:

• Non-wetting is the partial adherence of molten solder to a surface it has contacted; it leaves base metal exposed after testing.
• Dewetting is when molten solder coats a surface and then recedes, leaving irregularly shaped mounds of solder separated by areas covered with a thin film of solder; the base metal is not exposed.
• Pinholes are imperfections in the surface in the form of small holes that penetrate entirely through the surface.

For samples to pass the test, the critical area of the selected leads or terminations must exhibit at least 95% wetting. For exposed pad packages, the critical exposed pad surfaces must have at least 80% wetting. For IPC J-STD-002, this applies to all leads or terminations, unless otherwise agreed between customer and supplier. For MIL standards, this typically applies to 22 leads or terminations from a minimum of 3 test samples. If the sample fails, another set of test samples may be selected for re-testing, or an additional number of leads or terminations may be added to the inspection sample as agreed between customer and supplier.

FACTORS THAT AFFECT SOLDERABILITY

Several factors affect solderability, including surface finish and shelf life, storage conditions, oxidation and intermetallic compound growth, and contamination. Surface finishes and storage practices are the two most common starting points.

Surface Finish and Shelf Life

The surface finish applied to component terminations significantly affects both initial solderability and practical shelf life. HASL (Hot Air Solder Leveling) gives a robust, solder-compatible surface with a long shelf life but an uneven topography. ENIG (Electroless Nickel Immersion Gold) provides a flat, planar surface well suited to fine-pitch components, but it is sensitive to nickel corrosion. OSP (Organic Solderability Preservative) is a thin organic coating that protects copper but degrades with handling and thermal exposure. Immersion Silver and Immersion Tin both perform well when fresh; however, Immersion Silver is prone to tarnish, and Immersion Tin to intermetallic growth that consumes the tin layer over time. Each finish has distinct advantages and disadvantages and ages on its own schedule.

Storage Conditions

Temperature, humidity, and atmosphere drive the rate at which solderability degrades. Higher humidity and higher temperature accelerate oxidation and corrosion. Storage conditions adequate for six months are frequently not adequate for six years. Packaging materials matter — the use of moisture barrier bags with nitrogen, and preferably argon for very long periods of storage, is highly recommended.

Oxidation and Intermetallic Compound Growth

Oxidation is a surface phenomenon. Exposed metal forms an oxide layer that inhibits wetting. Flux is designed to reduce that layer during soldering, but once it exceeds a certain thickness, the flux activity available in a normal assembly process is insufficient, and the solder will not adhere.

Intermetallic compound growth is a bulk phenomenon. At the boundary between the solderable layer, usually tin, and the base metal beneath it, an intermetallic compound forms and grows. As it grows, it consumes the solderable layer. When the intermetallic reaches the surface, no solderable material remains to wet, and any joint that does form is brittle. This is why a component can look perfectly acceptable and still be unsolderable.

Intermetallic compound growth can be minimized by using plating barriers such as nickel, or reduced by implementing techniques such as cool or cold storage.

Contamination

A contaminated solder bath with excessive gold or copper, for example, can cause solderability testing to fail. This is why it is important to control and monitor contamination in the dip and look solder bath in accordance with IPC J-STD-001. Surface contamination such as flux residue or fingerprints can also cause components to fail the test.

WHY SOLDERABILITY TESTING MATTERS

Poor solderability produces solder joint defects that are expensive to find late and expensive to fix. Non-wetting and dewetting are the two most common — solder that never adheres, and solder that adheres and then withdraws. Both leave a mechanically weak joint that can pass a casual visual inspection and fail later under thermal cycling or vibration.

In electronics production, the real cost sits in the sequence. Screening incoming components is a controlled expense. Finding the same problem after the components are populated means costly rework. Finding it after the module is built often means replacing complete electronic assemblies. Early identification of solderability failures saves significantly compared with replacing finished hardware.

Solderability testing serves as a quality-control gate. It detects surface oxidation, contamination, plating defects and coating degradation before components reach the final assembly line, and it identifies the specific conditions that lead to poor solder adhesion — while the only thing at risk is the sample.

For high-reliability applications, the calculation changes again. Reliable solder joints determine the assembly's long-term performance, and in space and defense hardware, there is no service call after delivery. Failure is not an option.

Solderability Screening for Stored and Legacy Components

Components pulled from long-term storage, from bonded stock, or from the obsolete market arrive with an unknown surface condition. Solderability screening confirms those components remain assembly-ready and will not fail at next-level assembly because of an un-wettable oxide layer. This is one of the most common reasons components are sent for solderability testing.

Send us the component part number, the lot size and the applicable standard, and we will guide you as to which method applies.

WHAT CUSTOMERS RECEIVE

Each solderability test is documented in a written test report that identifies the components tested, the standard and method used, the preconditioning performed, the test parameters, the number of test samples, the inspection results against the acceptance criteria, and a clear pass-or-fail determination. The report includes photographic documentation of the inspected leads or terminations.

Solderability testing is a destructive test. Test samples must not be installed in flight or high-reliability final assemblies. Plan to send additional units specifically for testing.

There is no minimum lot size or minimum order for solderability testing.

SOLDERABILITY TESTING AT SIX SIGMA MICROELECTRONICS

Six Sigma Microelectronics has performed microelectronics soldering for forty years for commercial, defense, and space programs where reliability and precision are nonnegotiable. Solderability testing complements the services it most often supports — Column Attach, Ball Attach, and Hot Solder Dip — and the analytical capabilities Six Sigma uses when a result needs to be understood rather than simply recorded: 3D X-ray, acoustic microscopy, scanning electron microscopy, and metallographic cross-sectioning. That combination matters when a component fails. A solderability test tells you a component did not wet; cross-sectioning and SEM tell you why, which is the difference between rejecting a lot and fixing a supply problem.

Qualifications

Six Sigma Microelectronics holds DLA Laboratory Suitability, which covers solderability testing. Six Sigma is also DLA QML certified to Class Q, V, and Y, is AS9100 and ISO 9001 certified, and operates an ESD control program to ANSI/ESD S20.20. The DLA Laboratory Suitability certificate and letter are available for download from the Certifications page.

Turn-Time

Turn times vary with current factory loading, incoming order volume, and customer component availability, so we do not publish standard turn times. Expedited service may be available on request and is accommodated based on capacity and order priorities. DX- and DO-rated orders receive priority.

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