Solderability Testing - Dip and Look method; Wetting Balance Analysis

Solderability Testing

Solderability Testing pertains to the process of evaluating the solderability of terminations (i.e., component leads, lugs, terminals, wires, etc.). Industry standards for performing solderability testing include the following: 1) Mil-Std-883 Method 2003 - "Solderability"; 2) IPC/JEDEC J-STD-002 - "Solderability Tests for Component Leads, Terminations, Lugs, Terminals and Wires"; 3) IPC/JEDEC J-STD-003 - "Solderability Tests for Printed Boards"; 4) JESD22-B102; and 5) Part 21 of the IEC 60749.

The solderability of a surface is defined by its solder wetting characteristics. Solder wetting pertains to the formation of a relatively uniform, smooth, and unbroken film of solder that exhibits excellent adherence on the soldered surface. Non-wetting, on the other hand, is the condition wherein the solder coating has contacted the surface but did not adhere completely to it, causing the surface or a part thereof to be exposed. Dewetting is the condition wherein the solder recedes after coating a surface, creating irregular mounds of solder, but leaving behind no exposed areas.

There are several ways by which solderability testing is done, but the two most commonly used methods are: 1) the Dip and Look Method; and 2) Wetting Balance Analysis. In both of these tests, the samples undergo an accelerated 'aging' process before being tested for solderability, to take into consideration the natural aging effects of storage prior to board-mounting.

The Dip and Look Method, which is widely used in process QA and reliability monitoring, is a qualitative test process, i.e., judgment on whether a sample passes or fails the test is based on the physical and visual attributes that it exhibits.

Wetting balance analysis, on the other hand, is a quantitative test, i.e., it measures the wetting forces imposed by the molten solder on the test surface as it is dipped into and held in the solder bath as a function of time and plotted. The plot starts with the wetting force being negative (non-wet condition), which rises until it crosses the zero axis of wetting force, indicating that wetting has occurred. The time it takes for wetting to occur is one parameter used to assess solderability. There are, however, no established industry-standard pass/fail criteria for wetting balance analysis, which is why it is used primarily as an engineering tool and not as a production monitor. Wetting force depends on the density and surface tension of the solder.

Mil-Std-883 Method 2003 - Solderability Testing

The Mil-Std-883 Method 2003 is one of the oldest and most widely used standards for solderability testing. It will be used as the basis in the discussion below to provide more details on how solderability testing is normally done.

Mil-Std-883 Method 2003 employs the Dip and Look Method, requiring the following equipment: 1) a solder pot of sufficient size that can maintain solder at a specified temperature; 2) a dipping mechanism capable of controlling the rates of immersion and emersion, as well as dwell time, of the terminations; 3) an optical and lighting system that facilitates inspection at a minimum magnification of 10X; and 4) steam aging equipment for 'aging' the samples prior to testing.

The general solderability test procedure consists of the following steps: 1) proper preparation of the samples as defined by the acquisition document, but which must not include wiping, cleaning, scraping, or abrasive cleaning of the terminations to be tested; 2) aging of the samples in a steam ager, which consists of exposing the surfaces to be tested to water vapor for 8 hours and drying them either by baking at 100 deg C for no more than 1 hour in a dry atmosphere or air drying them at ambient temperature for a minimum of 15 minutes; 3) proper application of flux to the terminations; 4) solder dipping, which consists of immersing the terminations in static solder at a uniform temperature of 245 +/- 5 deg C; and 5) examination of the terminations at 10-15X.

The main criterion for acceptable solderability is 95% coverage of the dipped portion of the terminations by a new and continuous solder coating. Thus, pinholes, voids, porosity, nonwetting, or dewetting must not exceed 5% of the total dipped area.

Reference: Mil-Std-883 Method 2003 (please see the said spec for complete details)

Reliability Tests: Autoclave Test or PCT; Temperature Cycling; Thermal Shock; THB; HAST; HTOL; LTOL; HTS; Solder Heat Resistance Test (SHRT); Other Reliability Tests

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