Designing for wave solderability: board layout is the leading cause of solder defects, but getting it right is not easy.

Component suppliers' suggested layouts, in general, assume that only the supplier's particular component is used. In the real world, this is seldom the case. Often then, even when using "recommended" layout rules, the result is poor soldering. For better outcomes, one must recognize the effect of other components on the same board, and add common sense to the layout rules.

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The layout should promote solder wetting on joints and prevent solder bridging between joints. This may sound obvious, and easy, but it is often difficult to achieve because both wetting and bridging originate with the same physical laws--e.g., capillary forces--acting on solder during the soldering process. Capillary forces depend on dimensions and surface tension.

Small capillary gaps have a strong tendency to hold the solder because the capillary force in these cases is strong. If component leads are closely spaced, (e.g., SOICs), solder bridging is likely to occur, unless action is taken to promote solder drainage.

If components are densely packed and with closely spaced leads, they may promote solder bridging because of their influence on solder drainage at the point where the board exits the solder wave. The components' type and shape are also factors, as they may interfere with solder drainage conditions.

SMDs. SMDs are designed for reflow soldering. If one applies reflow layout guidelines to wave soldering, one may expect problems. Be sure to use the proper guidelines, especially with regard to SOT-23 pads, which are often too small. Be aware that the advised spacing, or free area, between components is valid only if they are surrounded by like parts. This means that if larger or higher components are mixed with small components, free area of the largest component must be used. This free area is necessary to promote wetting and prevent bridging. If this free area is not specified, leave an area around the component that is equal to or larger than the component thickness (height). With a mix of high and low components, the free space should be at least twice the thickness of the larger component to prevent shadowing during soldering, which might produce skipped (non-soldered) joints.

The space between adjacent pads of different components should be at least equal or larger than the thickness (height) of the thicker component. All SMDs should be placed so that solder flows easily along the joints. This means that SOICs should be placed lengthways to the transport direction, with sufficient large solder thieves on the trailing edges. Resistors, capacitors, MELFs, etc., should be placed perpendicular to the transport direction so that solder flows along both connections of the component at the same time.

Leaded components. Design rules for conventional leaded components mounted in through-holes are different for boards without hole metallization, as compared to boards with metallized holes. The design for nonmetallized holes is often more critical because air is not solderable, but with non-plated holes, the air gap between the pad and lead must always be bridged to make a full joint. As long as this air gap is smaller than 0.3 mm, a closed joint can be made, provided certain other requirements are fulfilled:

* The solderability of lead and land on the board must be perfect.

* The hole edge must not be damaged; e.g., by the inserted lead.

* There may be no foreign particles present between the lead and hole.

* The component might not cover the hole on the component side, and thus block vapors from escaping during soldering.

The pad size must be sufficient to give a good adhesion between pad and board. This does not mean the full pad size is needed to make a solder joint. It is often more desirable to increase the space between adjacent joints to prevent solder bridging because large pads promote bridging.

For metallized holes, the pad size should be minimized to prevent bridging between adjacent joints. A pad size of 1.3 mm will be sufficient in most cases. The solder joint gets its strength from the hole metallization.

Massive or "fat" solder joints. The visual quality aspects defined in several standards were established at a time when almost all solder joints were made by hand with a soldering iron. Solder joints are judged by the shape and wetting angle of the solder fillet. This quality control aspect was used for hand-soldered joints to prevent bad joints from being "buried" with solder. In machine soldering, however, it is impossible to get (much) solder on a non-solderable or difficult-to-solder pad or lead. So, if there is a large quantity of solder on machine-soldered joints, there is no question about the solderability. The "fat" solder joint is caused by solder drainage conditions and should not be reworked, as it is a perfectly reliable joint.

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Although proper design rules can prevent solder failures, it is difficult to get it right the first time. Board layout is the most important parameter for solder defect control. This can be proved simply by checking the defects: They will always be found in the same spots. If the soldering process was out of control, more random failures would be found over the whole board. The fact that failures are found in specific places points to the design.

Gerjan Diepstraten is a senior process engineer with Vitronics Soltec BV (vitronics-soltec.com); [e-mail]. This column appears monthly.