Ideal iron temp for center pin of LMR400

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KD2PII, Aug 2, 2020.

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  1. KD2PII

    KD2PII Ham Member QRZ Page

    I know for the shield I want to use a big high wattage iron with lots of thermal mass.

    However for the center pin I want to use a Weller WESD51 temp controlled iron. I'm just not sure what the ideal temp setting should be.

    Is 750F too hot? Something like 600 more appropriate? Obviously I want the solder to flow fast but I don't want the center conductor to get so hot that it starts to move around in the foam dielectric.
  2. AK5B

    AK5B Ham Member QRZ Page

    I do not know offhand the best temperature for soldering the center pin (if there is a best one) but I use a Weller D550 gun that heats quickly. The idea is to take a minimum amount of time soldering before the dielectric has a chance to melt.

    What I generally do is count to ten once I pull the trigger and then apply the gun tip at the tip of the center pin and touch the center conductor with thin solder. Release the gun as soon as it flows and then blow on the tip several times to effect a little cooling.

    TIP: Use Oatey water-soluble paste flux even if the solder used has a rosin core---works wonders every time! I dab it on with a toothpick or cable tie or whatever's handy.


    KP4SX likes this.
  3. KB0MNM

    KB0MNM Ham Member QRZ Page

    You failed to say: 1. Which type of solder you intend to use? Because of RoHS ( hazardous materials reductions- an international effort )- the actual melting points vary for the solder itself. That said, the thermal mass of the soldering element in the WES-51 tip is probably insufficient. The temperature can be set initially, yet it will change when you contact the heavy center pin and attempt to apply solder. I also use a 'gun' type of iron when I do solder a 'heavy coax' to a PL-259 or SO-239, rated for a higher wattage. 2. Which variety of LMR-400? There is LMR-400UF for ultra-flex, and other types for burial, etc. While I do applaud use of better cable for UHF frequencies, I also generally select a better connector. That brings me to: 3. Which connector pin do you plan to apply? The manufacturer of the connector may be using a Delrin(tm) insulation or something else. The 'pin' size varies more according to the actual connector in use: Type 'N', UHF (PL-259), UHF ( SO-239), mini-uhf, TNC, etc. While the minimum time will always benefit the dielectric, you also need to try to get a 'full melt' of the solder to prevent crystallization. That is one reason why some folks will only crimp connectors- your need for 'commercial-class results' may be lesser than theirs.
  4. KD2PII

    KD2PII Ham Member QRZ Page

    60/40 solder LMR400 UltraFlex Pin has PTFE insulator
  5. K7JEM

    K7JEM Ham Member QRZ Page

    On LMR400, the bigger issue is soldering the shield. You have to get it so hot that the dielectric will usually melt badly. This can cause a short. Less problem soldering the center conductor, much less mass equates to much less heat required, and it will cool down much faster.
  6. K1LKP

    K1LKP Platinum Subscriber Platinum Subscriber QRZ Page

  7. W9GB

    W9GB Ham Member QRZ Page

    IDEALLY you SET you Temperature from 650° to 750° F,
    depending upon the Solder Alloy used, and Flux level (Kester “44” is RA flux).
    Common Novice/Apprentice Error with Variable Temperature Solder Stations.
    Turn Up the Temperature Control (Instructors often reference it as the “Idiot Knob”)
    Soldering is part of the Skills: Brazing and Welding.

    LARGER MASS ... MATCH the SOLDER TIP Size to the Work (Simple thermal concept).
    Jacking the temperature on a small mass tip, produces Poor Joints and Shortens Tip Life.

    For your LMR400 example, the Copper is a THIN VENEER over Steel.
    I would look at the larger Screwdriver (Chisel) Tips:
    Such as an ETB (3/32” width); ETC (0.125” width) or ETD (0.187”).

    Solder Alloy : Melting Temperature Alloy Melting Temperature.pdf

    KESTER, an Illinois Tool Works (ITW) Company
    Alloy Temperature Chart
    Commonly specified solder alloys are shown in the table. The selection of alloy is determined by application, melting temperature and physical properties. The alloys listed may be available in forms other than those indicated. Other solder alloys are also available. Base/Alloy Temperature Chart.pdf
    Last edited: Aug 2, 2020
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Anything over 500F will melt the solder just fine, but thermal mass provided by a large chisel tip iron or the ability to pump a lot of power into a gun tip is very much preferred to get "on and off" the work very quickly.

    You could set a small-tip soldering pencil to 900F and it won't do the job well, since as soon as you touch it to the center pin of a PL-259 with the LMR400 center conductor installed, that mass will cool the pencil tip down very quickly, requiring more time to apply more heat. That's not preferred.

    Whatever allows you to flow solder through the connection smoothly and quickly is preferred.

    I use a Weller SP-120 with a 1/2" chisel tip even for the center pin contact. The thermal mass of that huge iron is so much larger than the mass of what's being soldered, that solder flows within a second or two, and within 3-4 seconds the job is done and I'm off it.

    A powerful gun can do this, also, while drawing a lot more current from the mains.

    When I used to make a lot of coaxial cable assemblies with solder connectors back 20-25 years ago, for "quick on and off" with LMR400 and similar easy-melt dielectric cables I used a 900W resistance welding station, which I still have but don't really use anymore. That had dual foot pedals, one to lower the upper electrode to the fixture where the connector sat and the other to apply power. That could melt solder through all four soldering holes in a PL-259 body in a couple of seconds, and then removing the power caused the big copper electrodes to become a heatsink which cooled it off very quickly; you could have the body so hot solder would flow anywhere on it instantly, and then cool enough to just pick up in your hand a few seconds later.

    Perfect for this job! But nobody would install such a machine just to solder a few connectors.
    W6MK likes this.
  9. KD2PII

    KD2PII Ham Member QRZ Page

    @W9GB yeah i have kester 44 both as 60/40 and 63/37. I agree larger mass is good but wasn't sure how much would overheat the center conductor.

    @WB2WIK - with the amount of strife these connectors give me i probably would install such a machine if i had one.

    I tried using the "clamp" style PL-259s mentioned here and they work great for HF. They're even okay at VHF. But on 70cm the SWR starts to go up.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    "Mass" doesn't overheat the center conductor, temperature*time does. Irons with much larger mass can actually operate at lower temperatures and be less destructive because the "time" element is much shorter.

    Pb/Sn alloy electronic solders like you have all melt at well below 500F.

    Re the clamp vs. solder PL-259s, none were really intended for operation at 70cm, but it's a happy coincidence most of them do; none of them are really 50 Ohm connectors, and a lot of the discontinuity is in the female receptacles, more so than the male plugs. I use PL-259s at 700W on 432 MHz and they work fine. Some loss, but it must be very little as the connectors don't get even slightly warm at 700W into a dummy load after 2-3 min of continuous carrier key-down. The cable (LMR400) gets a bit warm, and warmer than the connectors do.:p The big Bird load starts to feel a bit warm after a few mins, but not much -- it's very large, and oil filled with a lot of heatsink fins. Takes several minutes before you can really feel any heat from that.

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