Smith-Kettlewell Soldering Series Part 5

Table of Contents

Soldering I
Soldering II
Soldering III: Tinning Stranded Wire
 Soldering IV: Popular RF Connectors
Soldering V: RCA and Motorola Plugs
Soldering VI: Resistance Soldering
Soldering VII
JA3TBW Solder Guide
Soldering Kinks
Vinther Fingertip Soldering Iron

SOLDERING, PART VI

Multipin Connectors

This task is more demanding of your skills than any other. You can rest assured that any such connector has been designed to be of minimum size. Also, there will always be nearby wire insulation that you can abrade with the iron. Nevertheless, I have learned tricks which help--both in finding the pin and in avoiding others--some combination of which may get you out of a jam.

There are literally hundreds of varieties of multipin connectors, any of which you are likely to encounter. However, since the problems are similar (even for doing small switches), I have chosen two examples for this discussion. One is the DB25 connector (which has become the standard for computer interface), and the other is the DIP (dual in-line plug) that fits into IC sockets. If you can solder wires onto these, you can do anything.

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[At the end of this article, computer connectors are listed which permit soldering of the pins onto their wires before insertion, thus making this job much easier. However, these are specialty items, and your Radio Shack store won't have them.]

The DB25 computer connector has two closely spaced offset rows of pins; 13 on one row and 12 on the other. (Actually, these connectors come in sizes of 9,15,25,37, and 50 pins, the 25-pin animals being most common.) The back ends of the pins (or sockets, since the females are similar) are scoop-shaped; being tubular to start with, their ends are cut off at a steep angle to leave "scoops" into which wires are laid and soldered. The "inner diameter" of each scoop is just large enough to accommodate wire of 20 gauge, and they have no eyelets or holes through which the wires can be hooked. In short, each wire must be held in its scoop, the pin then heated with the iron, and solder deposited into the scoop to surround the wire.

The DIP plugs are made up of standard flat IC pins mounted in a flat insulator plate; the top ends of these pins protrude only 1/16 inch above the insulator board, and these back ends are slotted to form little "forks" into which the wires are soldered. (Up to

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22-gauge wire can be accommodated.) Each wire is laid in its fork so that its end reaches out- toward the side of the plug. It is then soldered to the fork, after which the end of the wire is clipped off close to the pin.

It should be obvious from the above descriptions that good systems for holding these items should be instituted. Vises and holding clamps are not only required for the connectors, but also for the wires and cables. For- example, one relatively heavy vise would be used to hold the connector. A smaller vise can be used to hold the cable (perhaps a foot back from the connector), and a nearby alligator clip can be set up to hold the individual wire, not more than an inch back from the connector. (These are only suggestions, but something has to keep the little wires in contact with their "scoops" or "forks.")

[Holding the individual wires will probably not be necessary in cases where the traditional solder lugs, having holes through which the wires can be mechanically secured before soldering, are present. In these cases, bend the stripped and tinned end of each wire almost double to form a "hook"; hook the lug with the wire and squeeze the

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hook closed with needle-nosed pliers. As the wire now holds itself in place, no complex array of clamps will be necessary. However, you will thank yourself later if you hold the cable so that wires come straight back off the pins, rather than at odd angles; they will be easier to trace in the future, and bridged wires will be less likely.]

Clamping Arrangements

The vise for holding your connector should be able to swivel and tilt in several directions. As the thicket of wires becomes significant, you will want to position the connector so as to avoid marring previously done wires with the iron. Also, depending on the connector's design, soldering on female terminals may have to be done with the pins horizontal, so as to keep solder from running through the terminal and ruining the socket. You will often have to use your imagination in order even to grab a connector in the vise. For example, there is not enough meat on a DIP plug to hold securely. In this case, the plug can first be inserted into a good, deep socket to form an assembly of significant substance to be clamped securely.

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A good vise for holding the connector is the "Panavise" listed in the "Tools" section of Soldering, Part II (SKTF, Winter 1981). (Of particular usefulness are a swivel-type base such as the No. 380 "Vacuum Base" and the standard vise head, No. 303.)

Unfortunately, however, the jaws of the Panavise are plastic; they will readily contaminate the tip of the iron if struck accidentally, and they are easily marred by the iron. For this reason, it is advisable to line the jaws with braille paper, leaving flaps of paper folded back over them to protect the jaws from the iron.

I have used an extraordinary variety of arrangements for holding individual wires behind their terminals. An alligator clip suspended by a piece of coathanger makes an adjustable structure for this purpose. The far end of the coathanger can be held in another clamp (such as your board clamp). Some arrangement with locking forceps in a board clamp is also effective. Actually, the wire being worked on can be fixed to one which has already been done by biting them both in a small alligator clip. Holding a wire parallel to its neighbor is a rather secure way of doing it.

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[There are commercial holding devices which are built around alligator clips. One such instrument -is the Radio Shack 64-2093, known as the "Helping Hands." Mounted on a small cast iron base, ball joints are used to support two alligator clips, one at either end of a horizontal boom. The base of the "Helping Hands" is not very heavy, however, and building a new base for it would be highly advisable.]

Any arrangement for holding the cable a foot away from this paraphernalia will work. Very often, I will just sandwich it between a couple of heavy books or transformers; tying the cable to a C-clamp on the edge of the workbench is ideal. However you do it, the idea is to keep the cable from pulling back or twisting as you set up the clamping system for the connector.

Preparing the Cable

Short leads cannot be formed or held so as to stay in position for solder. Furthermore, arranging leads to your advantage cannot be done without flexing the cable; short leads that were previously attached will be stressed to the breaking point. Finally, if heat-shrinkable tubing is threaded over each

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wire in anticipation of insulating the finished connections, short leads will conduct enough heat to the tubing to shrink it while soldering--it will no longer accommodate the terminal. Therefore, the prepared ends should be at least 1-1/4 inches long.

After you have stripped and tinned the ends, inspect them for marred and peeled insulation. Cut the stripped portion very consistently to perhaps 3/16 inch in preparation for attaching the wires to their respective terminals.

[Of course, the appropriate length of the stripped and tinned ends depends entirely on the type of solder terminals on the connector. For example, large solder lugs with holes in them demand that 1/4 inch of lead be available to hook through them. Also, in the case of DIP plugs where you are soldering to very tiny forks, you may wish to make the bare ends quite long (perhaps 1/2 inch) so that you can use this wire as a landmark to guide the iron. In any case, you will find that consistency is your best friend; errors are quickly spotted among a grove of otherwise uniform terminals.]

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Soldering--Doing More Good Than Harm

Probably the single most significant tool to enhance the chances of successful work on multipin connectors is the Japanese Solder Guide (the JA3TBW Solder Guide, SKTF, Spring 1983). Invented by Mike Bhagwandas of Kobe, Japan, it provides the user with a good means by which a desired terminal can be found with absolute certainty. A brief description follows (appropriate suppliers are listed at the end of this article).

The solder guide consists of a 3- or 4-inch piece of thin-walled stainless steel tubing whose inner diameter is about 0.05 inches. Flux-core solder of about 0.03 inches in diameter is fed through the tube so that it just emerges from the "bottom" end, this bottom end is then rested on the desired terminal. In operation, the tube is used to guide the hot iron to the terminal, and solder is then fed to the work from the "top" end of the tube.

A sort of "handle" can be provided near the center of the tube, this can be made from a drilled-out braille stylus handle, or a simple shaft collar can be secured to the tube with a setscrew.

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Solder is fed down the tube with the thumb and first finger, while other fingers are used to support the tube above the "handle." The iron is brought to the lower section of the tube, whereupon it is slid down to contact the work pieces.

Soldering can still be done the old way (without the tubular solder guide) by using tactile feedback to verify that the right terminal has been found with the iron. without the tube, a system of nearby landmarks will greatly enhance your chances of hitting the right point with the iron; these landmarks might consist of, first, the vise, second, a nearby alligator clip, etc. With the solder in one hand and the iron in the other (the solder resting on the work), verification of hitting the terminal with the iron will come by noting the vibration of the solder, then by watching the solder melt.

Bridged Connections--

Because the pins are close together, a common error is the accidental bridging of two adjacent ones. Shorts between pins can occur in the following four ways:

I often lose the pin temporarily with the solder--either I knock it off with the iron, or it melts before I have expected it to--and

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I must then fish around with the solder to find the hot pin. In re-finding it, it is easy to spill a little extra solder into the connector, usually on the pins below. This is, however, one of the easiest mishaps to correct. If metals onto which solder has been spilled never reach soldering temperature (they usually will not if they receive no direct heat from the iron), solder droplets will not firmly adhere to them. The droplets can be dislodged later with a braille stylus or a probe-type soldering aid. Remember to check for droplets if you suspect that spillage has occurred.

Icicles of solder often form if the iron is abruptly pulled away from a connection-sliding it off the connection will usually break the surface tension and prevent icicles. If you abruptly leave the pin in the direction of an adjacent one, you can form an icicle which decreases the distance between pins. On the other hand, there really is not room to talk about sliding the iron off these closely spaced terminals. Therefore, go ahead and jump off the pin--knowing that occasional icicles will result--but do so in a benign direction (straight up, for example).

If wires are touching each other directly behind the pins, their insulation will some-

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times melt from transmitted heat, whereupon the insulation will give way and allow the wires to touch each other. If this happens, they can often be pried apart and then reinsulated with tape (or tubing, if it has been previously installed onto them).

The most sinister kind of bridge is the bonafide "solder bridge"; this occurs when two adjacent pins are simultaneously heated, thus allowing a bridge of solder to alloy with both of them. To clear this type of bridge, the connections must be simultaneously reheated and a probe of some sort passed between them. [Sometimes a knife or a small "mill file" (a mill file has teeth on its edge) can be used to destroy the bridge without reheating the pins, but you risk breakage of the pins or the wires with the inevitable rough handling.]

There are four ways of detecting shorted terminals. These are the indications you should look for:

  1. Connector pins often exhibit some mobility in their socket block; they can often be wiggled slightly. They should all wiggle separately--one at a time. If two pins insist on moving together, they're bridged.
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  2. Some kind of probe that fits between the terminals can be used to explore them--a stylus, a soldering aid, a dental tool filched from your dentist, a jeweler's screwdriver, etc. Make sure that you can feel the insulating material of the socket block at the base of the pins; then make sure that you can bring the tool unimpeded all the way back to the insulated portion of the wiring.
  3. If little pieces of tubing have been installed onto the wires in anticipation of insulating the pins (thus burying your sins, as my supervisor used to say), a good indication of success is if the little segments of tubing slide all the way down over the pins to the insulator block. Once again, consistency will be your best friend; if you have cut all the tubes to exactly the same length, unevenness at their back ends will show you which ones to investigate further.
  4. Finally, the best (and the most tedious) test of the connector is checking it with a continuity tester, going pin by pin. Each pin can be checked for shorts to all of its neighbors (four neighboring pins, in the case of the DB25); each pin can be tested for continuity back to its wire at the other end of the cable. (I don't always go through
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    this rigor myself. However, the electrical items of lowest reliability are connectors; if you really want to know that these are not your problem when the stuff doesn't work, check the connectors thoroughly.)

[For making the above continuity checks, special test probes that fit the connector are very helpful. These can be made by ravaging a spare or damaged unit of the opposite gender. First, remove any metal framework with a hacksaw--then dissect the insulator block with a coping saw to get a few sample terminals. Then, make test probes out of them, using heat-shrinkable tubing or tape to cover all but the business end. The probes can be plugged into the connector you are testing, and the tubing on the outside will keep you from getting false alarms when fishing around adjacent pins.]

The Paper Dam--

This trick has indeed served me well; I first tried it when I was called upon to wire connectors having 120 pins (a slice of my early work history that I would just as soon forget). My first "dams" were made from little strips of braille paper-this worked most of the time, but starting a conflagration is possible. Other materials worked better in the face of extreme temperatures.

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My favorite material is "fish paper," an insulating material that is not easily damaged by the heat. (GC Electronics of Rockford, Illinois markets fish paper in a 10- by 24-inch rolled-up sheet. Their name for it is "Fyberoid," No. 560.) Another good source of fish paper is the scrap heap; old TV sets and other consumer products are loaded with it. Though bits of it look like scrap to someone else, I save every square inch.

Another material is Teflon, strips of which can be cut from liners of broken tape cassettes. (Make sure that the plastic liner of your cassette is Teflon, other soapy-feeling plastics which are not heat-resistant are used in cheaper tapes.) You should be careful of Teflon, however; once it does reach the temperature of its destruction, it gives off gases which are highly toxic--keep a breeze going around the workbench.

You can test a material for its suitability by bringing the barrel of your iron into contact with it. If it melts or smokes, don't use it.

The principle of the dam is to isolate the terminal of interest from its neighbors. It will not only prevent bridging, but it will

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do much to keep you from marring previously done wires in your attempt to find the terminal with-the iron. The dam, a strip which is perhaps 1/4 or 3/8 inch wide, is "woven," as you might say, through the terminals.

My usual configuration is an L-shaped affair with the bend of the "L" being a small u-shaped gutter that accommodates the terminal being soldered. (In terms of items you might recognize, its shape is rather like a sewer trap.) Suppose, for example, you are soldering wires along the top row of pins on a DB25 connector--also picture yourself facing the end of the connector. Suppose also that one or more wires have been attached near the end farthest away from you. One end of the dam would be sticking straight up between your next terminal and the ones previously done. The dam then courses under the terminal of interest, after which it lies flat on top of the pins to come.

The dam then becomes your main source of landmarks with the iron, you can follow the horizontal surface of the dam until it drops down under the terminal you want. Or, with the iron, you can first find the vertical fin that prevents you from hitting previously done wiring--then follow this vertical surface down to the pin. In any case, there is

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only one pin to which you have easy access, and it's the one you want.

A much simpler example is that of using a dam for doing a pin at the end of the row. Although I seldom use one here, a simple vertical strip between this and the other pins is sufficient.

If the pins are long (which they are not in the case of the DB25 connector), they alone will hold the dam in place. If they are short, someway of holding the dam in position will have to be devised. The fish paper dam can be held in place by a small bit of masking tape; lay the tape across the horizontal leg of the dam and secure it to the frame of the connector. Teflon, on the other hand, resists adhesion to sticky surfaces, and it is best held in position by a small loop of wire or an alligator clip.

The kind of tape you use is very important. Don't put plastic tape around where you can accidentally find it with the iron; it will soil the tip and render the soldering iron inefficient. On the other hand, no adhesive tape is impervious to the heat of the iron. However, I have had good luck with paper tapes (such as masking tape), and you should apply it with the understanding that it

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should not be used for resting the iron against while soldering.

[In reviewing my procedure in preparation for writing this article, I came across a DB25 connector of unknown brand whose insulator block is easily melted by the iron--a sinister practical joke indeed. My only recourse was to lay masking tape over the side of the connector with the edge of the tape "lipping" down over the insulator block above the pins. Heating the tape was unavoidable, but it was better than soiling the tip of the iron and ruining the connector block. ]

Additional Comments on the DIP Plug

Because of its construction (having very short pins to which wires are soldered at right angles), I have found no way to isolate these pins from one another while soldering. Success depends mainly on how stable the clamping setup is--whether or not the wires tend to stay in position while you work.

The wire of interest can serve as a good landmark for finding the pin. If previously done wires are cut off close to the

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terminals, and if a substantial length of your prepared lead is allowed to protrude past the edge of the plug, this lead becomes "the only tree in the forest." The iron can be used to find the lead while the solder is held against the pin to detect vibration when doing so.

If you wish to take the trouble, there is a very secure system of holding wires in position which I have recently tried. Insert the DIP plug through a piece of perforated board, then plug it in to a good-quality socket to hold it there. When stripping and tinning each lead, provide one full inch of bare wire beyond the insulation. Before laying the wire in its fork, poke it into a hole in the perforated board adjacent to its pin and bend it over so as to hook it in position. Then stretch the wire across the fork and tape it down against the board on the other side of the socket. With this system, jostling the wire loose from the fork will be unlikely.

After one row of pins has been completed, you will find that avoiding this wiring while doing the second row is very difficult. This again is a good application for masking tape place masking tape across the completed forest of wires to protect them from the iron.

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Miscellaneous

This article has been very difficult to write--connectors are widely different, and there is no foolproof procedure to assure successful soldering. All I can do is suggest hints and techniques which have been of use to me in various circumstances.

As you develop your own methods, you will no doubt sacrifice connectors. Always buy extra ones, knowing that spare parts from the old ones may be useful as test probes, and in some cases may be used to replace damaged terminals.

Where possible, do your soldering before inserting the terminals into the socket block, there will be no nearby terminals with which you can bridge connections, etc. A good example of this was the 120-pin connectors I was called upon to solder in my work as a technician. After wiring them (and having much trouble with the finished devices), I discovered that my supervisor had inserted the terminals into the blocks--he was trying to be helpful. Wherever those instruments are, the connectors are still a source of trouble.

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Pin Connections

It is customary to number connector pins, viewing them from the pins of males or from the backs of female sockets. In addition, if a "key way" exists (some mark around the perimeter of the connector), this key way will generally appear between the lowest and highest numbers. For example, DIP plugs will usually have a notch at one end, holding the plug with the notch up and with the male pins facing you, count them in a clockwise direction starting with the pin to the right of the "key way." A 14-pin DIP plug would have its key way placed between pins 1 and 14, the column of pins on the right-hand side would be numbered 1 through 7, while the pins on the left side would be 8 through 14 (7 and 8 near the bottom).

The DB-type computer connectors are numbered a bit differently. For example, viewing a DB25 connector with the male pins facing you, orient the rows of pins so that the row of 13 is on top. Then, pins 1 through 13 are on the top row (counting from left to right), pins 14 through 25 are on the bottom row (also counting from left to right).

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The Amphenol "Poke-Home" Series

These units are shipped with the pins not being installed in their socket blocks. As you can imagine, soldering these pins individually would be much easier than soldering pre-assembled connectors. When soldering, the forward ends of the connectors should be held in something which does not act as a big heat sink (they should not be directly held in a metal vise). Line the jaws of your vise with braille paper or fish paper before grabbing the pins. Another idea is to hold the pins in something of less heat capacity, such as gripping them in your locking forceps.

The numbering system is simple. For example, the male set (insulator block and 25 pins) of a DB25 plug is an Amphenol No. 17-20250. The female set is an Amphenol No. 17-10250. Connectors with a different number of pins require the "25" figure to be changed. For example, a DB15 male would have the number 17-20150, and a 9-pin male would bear the number 17-20090. In like manner, a 37-pin female would bear the number 17-10370.

Several people make equivalents to the "Poke-Home" series, and it is always worth asking your supplier for this breed. However, a more common version of the "Poke-Home" series has crimp-on pins, and an expensive crimping tool is needed to attach them. Be sure you know what you're getting.

Suppliers

Solder Guide Parts--6-inch lengths of hypodermic tubing, catalog No. HTX-15, are available from Small Parts, Inc., 6901 North East 3rd Avenue, Miami, FL 331381 (305) 751-0856.

Shaft collars are available as "set screw collars," catalog No. 889, from the Player piano Company, 704 East Douglas, Wichita, KS 67202; (316) 263-3241.

Amphenol Industrial Division, 1830 South 54th Avenue, Chicago, IL 60650; (312) 242-1000.

GC Electronics, 400 South Wyman Street, Rockford, IL 611911 (815) 968-9661.