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Reprinted from the EIC/EMCW report of the proceedings.

In recent years, a growing demand using transformer bobbins with pins as leads, has combined with the growth of the worldwide industry. As such, many manufacturers are purchasing, producing, and supplying product to many and various end user, industry, and military specifications.

This paper will address various base metal and plating considerations for wire used as bobbin pin leads.

All wires produced today are produced to specifications. Wire is measured and compared to selected production specifications to see if it was produced correctly, so   it can perform properly in the application. Specifications must be selected properly    and be capable of determining if the wire is functionally appropriate in order for the applications to perform as required.

A. Base Metal Alloys

Most bobbin pin materials come from the list of alloys and specifications below:        

These alloys have been selected for various reasons: chemistry, thermal conductivity, strength, formability, coefficient of expansion and availability. (See Table 1.) Alloys     are created typically for solid solution strength at solution phase optimums. Increased copper per centages provide increased conductivity; increased nickel provides     strength, lower expansion rates and elevated temperature service. 

B. Tempers

Although the tensile limits in these alloys reflect the Ultimate Tensile Strength capable of being achieved the extra hard and spring tempers are mostly used when producing straight pins. Otherwise, pins which have to be formed, or upset with retention features, or coined or swaged into tandem pin (end-to-end) configurations are specified from the much softer and more formable 1/2 Hard, 3/4 Hard or Hard tempers. (See Table 2.) Serious consideration should be given to the trade-off of formability and strength.

C. Cross-sections

Present designs are utilizing wire for pins with round, square and rectangular cross-sections. Mechanical property specifications for square wire are for the most   part, the same as that for round wire. Rectangular wire (or flat wire) will be considered mto have round/square mechanical properties when sizes have width-to-thickness   ratios is 3:1 or less. Wires which have ratios which are greater than 3:1 will use  industry specifications more typical for strip and/or stamped terminals.

Table 1.

Significant in the specifying of square and rectangular wire is that of the wires diagonal or corner radii. Specifying dimensions, along with total tolerance, (square, diagonal or corner radius) aids in refining control over pin size and its retention characteristics.    The formulae are below:

When  D = Diagonal
           S = Square Size
           R = Corner Radius

           D = 1.4142 S - .8284 radius
           R = 1.4142 S - D
                      and
           S =  D + .8284 R
                       1.4142

D. Electroplating and Coating

In recent years, platers of tin and tin-lead finishes have been yielding to increase pressures to reduce lead content in the finishes, plant environment and plating waste affluent. More "environmentally friendly" percentages of 90/10, 93/7 and 95/5 tin/lead have replaced the more traditional 60/40 tin-lead and 100 per cent tin plating.All of the  tin and tin lead platings can be applied in various controlled thickness ranges.  Thickness specifications typically indicate a 3-5 micron minimum and are specified  both with and without additional underplatings of nickel and copper. The actual  thickness of the overplate is significant to the lngevity of solderability. Tin and tin-lead finishes are produced in bright, satin and matte finishes. However, specifying the    plating finish is a severely limiting detail in so far as this sonsideration may limit suppliers and supplier processing options.

       2. Underplating

The two most common underplating are nickel and copper. Specifications which   include underplatings are developed for many reasons; base metal isolation, convenience to the electroplater, and short and long term solderability concerns.

Copper as an underplate is chosen typically if nickel cannot be used. When it is used as a "flash" to 1 micron it is mainly employed as a vehicle to prepare the base wire (copper alloys) to be overplated.   When present in 2.5 microns or greater copper will operate as a diffusionary barrier allowing migratory base metal constituents to be absorbed in the highly soluable copper layer. Thus in this manner the migratory constituents do not reach the susceptible tin and tin-lead layer and cause problems      in the overplate ability to wet.

Table 2

Tensile ranges for Round and Square Wire
(ksi.)

E. Solderability concerns

F. Surface Treatments

In lieu of knurling the surface of the pin material, pin retention may be enhanced by coining or upsetting with securing star, flat or barbed feature. Some of this may be performed in tandem with insertion operations or performed separately.

CONCLUSION

REFERENCES

BIOGRAPHY

Jim McGowan graduated from Drexel University, Philadelphia, PA., Bachelors of Science. He has worked 11 years as a Sales Engineer in aluminum master alloys, tantalum, columblum and high strength nickel and nickel cobalt alloys and as a   product specialist in beryllium copper for Kawecki Berylco Industries, Cabot Corporation and NKG Metals Corporation. He has been employed for the last 10years at R&F Alloy Wires, Inc., Fairfield, NJ where he is vice president of sales and marketing.