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Reflow Soldering Process Considerations for Surface Mount Application

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INTRODUCTION

Reflow soldering, like wave soldering, is not a new manufacturing process. The hybrid industry has used and refined the art of reflow soldering for many years. However, with the advent of Surface Mount Technology (SMT), reflow soldering has expanded in the number of types and has been studied, refined and explored as never before.

Many different opinions have been expressed about the best process. We have found that the best or optimum process is the solder process which resulted in meeting the goals of reflow soldering for the SMT application. You can select the optimum solder reflow process for your application if you:

• Understand the goals.

• Understand the process and modify the process to meet the goals.

• Realize the best results are obtained from the process which treats your product with care.

GOALS

The goals for reflow soldering are of two basic types. The first is more traditional and includes:

• Uniform solder joints.

• Minimum repairs and part replacements.

• Minimum solder skips, solder balls and part movements (tombstoning, etc.).

• Maximum cleanliness of the completed assembly.

• Maximum flexibility to allow soldering a large number of circuits with minimum changeover time.

The second type of goal is not normally thought of by process engineering when first establishing the reflow solder process. These goals include:

• Minimum time above the liquidous solder temperature to reduce solder grain growth, resulting in a more durable solder joint.

• Minimum stress and damage to the Printed Circuit Board (PCB).

• Minimum damage and stress to the SMT parts.

• Minimum “leaching” of part termination materials.

• Optimum conditions to minimize movement of parts (all the neat names of tombstoning, drawbridging, etc.).

These goals are in line with increasing product quality and reliability.

To achieve these goals, it is important to under- stand the process, to modify it to meet our goals and to insure the product is protected. There are many technical and commercial descriptions of the reflow process already in print. Our look at the process will be a little different as we look at each phase, explain what it is attempting to achieve, show concern for important items, and point out some “do’s and don’ts”. Each of the specific processes will be re- viewed for its positives and what the critics are

saying about it. Methods of improving on the pro- cess, to balance these criticisms, are also dis-cussed. The aspect of treating the product with care will be interwoven in the discussion and further discussed for some specific cases.

Let us look into the process further.

REFLOW SOLDER PROCESS DESCRIPTION

The basic reflow solder process consists of:

• Application of a solder paste to the desired pads on a printed circuit board (PCB).

• Placement of the parts in the paste.

• Applying heat to the assembly which causes the solder in the paste to melt (reflow), wet to the PCB and the part termination resulting in the desired solder fillet connection.

A. Solder Paste

The solder paste mixes are improving as the demands of reflow soldering for SMT increase. Selection and specification of the optimum paste is a key item in the reflow solder process.

B. Placement of the parts in the paste is not difficult if the pad design considers all the applicable tolerances. (See KEMET Application Bulletin “Surface Mount - Mounting Pad Dimensions and Considerations”). Care should be taken during the transportation of the PCB’s not to smear the solder paste or move parts. Inspection of placement accuracies and subsequent manual movement of parts in wet paste has been shown to increase repair rates after soldering.

C. Application of heat to result in the eventual solder joint must consist of the following discrete items:

- Preheat cycle is intended to drive off most of the volatile solvents contained in the paste before the flux begins to work. This assists in initiating fluxing action on the solder powder and the metal surfaces to be joined.

- Additional preheat time to elevate the temperature of the PCB, solder paste, and terminations to a temperature near the melting point of the solder.

- Additional heat transfer to elevate the temperature over the liquidous point of the solder.

- Temperatures to be achieved are the liquidous melting point of solder. Liquidous points for -

60 Sn / 40 Pb solder is 188°C

63 Sn / 37 Pb solder is 183°C

62 Sn / 36 Pb / 2 Ag solder is 179°C Additional heat is required to insure activation of the rosins. However, heat should be limited to minimize the times some parts are above critical temperature.

- Assisted temperature cooldown to the solder solidification temperature, followed by gradual (static) cooling to temperature near cleaning temperature.

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