SMT: Solderability Testing, Edge Dip Method

5.1 Solderability testing, edge dip method:

Here the specimen is partly dipped vertically and edgewise into molten solder and then withdrawn, after which the quality of the coating is assessed visually. The dipping can in principle be done by hand, but an automatic dipping device affords more control of dipping conditions. Edge dipping is applicable to all electronic components, including those for surface mounting, and to substrates carrying metallized tracks. The parts to be dipped are clamped in a small stainless steel clip and remain in the clip while being fluxed and dipped in solder. After dipping, the solderability and resistance to dissolution of metallization are assessed visually. Immersion of the components is carried out with the areas to be examined not less than 2 mm below the (undisturbed) solder surface, but not more than is necessary according to the component shape. The speed of immersion and withdrawal are usually between 20 and 25 mm/s.: solder composition is tin60- lead40. Bath temperatures and dwell times are given in table below.

Solder temperatures and immersion times for dip testing

Note(1): Wettability is normally tested at 2350C, just below the lowest temperature of most soldering processes. The temperature of 2150 C allows testing at the relatively low temperature normally used in vapour phase soldering, which is important for some kinds of components having a metallization that does not melt at this temperature, e.g. IC packages with leads having a plating of pure tin or a tin-rich alloy. A one second longer immersion time is given, as the wetting reaction, even on a readily-wettable surface, can be expected to be slower.

Note(2): The edge dip method is perfectly appropriate for testing the occurrence of permanent wetting or dewetting. The specimen should be allowed to cool in air after withdrawal, without forced ventilation, as the development of dewetting usually takes some time.

Note(3): When testing resistance to soldering heat, certain large flat components (e.g. ceramic chip carriers), if immersed with the seating plane vertical, will not experience the same thermal gradient across their thickness that they would in practical soldering. In such cases, the floating attitude may be chosen by the component specification writer. Discrimination between different sizes of component by varying the immersion time is not considered desirable.

Note(4): In wave soldering, the speed of dissolution of metallization is much greater than in static dip; the component may, moreover, be subjected to iron soldering for touch-up or repair. A fairly long immersion at high temperature is therefore given for testing the resistance of the metallization dissolution is sufficient and the indicated immersion time is then too long for a practical assessment; in these cases a shorter time may be selected.

Solderability depends on 1. inherent character of the material concerned, 2. degree of cleanliness of the surfaces after the fabrication of the components, 3. aging during storage. The following precautions may be taken.

1. recommended metallic coatings, including requirements for minimum layer thicknesses;

2. thermal requirements for components and boards;

3. measuring methods and requirements for solderability of component terminations, printed boards.