TABLE OF CONTENTS In Sec. 5.7, we discussed the preplacement of flux and solder in the joint before heat application. The methods used are judged by their quantity control. The importance of solder quantity per joint is further explored in Sec. 10.3. Obviously, the economy of the process and the production size are additional selection factors. Finally, surface topography has a direct impact on the final assembly process.
A flat surface contour is easiest for most assembly techniques, while a rounded surface profile can cause mating-part slippage during assembly. Table 10.1 lists the most common solder-preplacement methods and rates them in these important issues.
| Method | Quality control | Contour outline | Process cost | Application comments |
|---|---|---|---|---|
| Paste or cream | Good | Best | Medium | Good on any surface and contour; tacky paste can hold parts together |
| Solder preform | Best | Flat | Medium | Need conforming surfaces and must juggle three parts |
| Hot dip coating | Poor | Rounded | Low | May not pick up enough solder in coating |
| Fused plating | Very good | Rounded | High | Normally limited amount of solder in coating |
| Electroplating | Very good | Flat | Medium | Has unpredictable aging solderabil-ity potential |
To summarize, flat surfaces are ideal for part assembly unless they have unique shapes designed into them. When a truly level exterior is not possible, a slight curvature is also acceptable. The use of a tacky paste or flux will help to hold the parts together during assembly.
There are two ways of supplying solder to the...
