TechNetters,

     Can any one shed some lights on my problems below :

     I've recently conducted a study on the strength of three types of
     plating on our products' surface mounting header pins which are
     reflowed on the pcb. Some details are as follows :

     ~Types of plating :

        Type A :base material is phosphor bronze, coated with 0.00005" Ni, then
                with 0.000015" Palladium/Ni(80/20), then 0.000002" gold flash
                outer layer.

        Type B :similar to type A above, except that the outermost gold plating
                is 0.000007" thick.

        Type C :phosphor bronze base material, coated with 0.0001-0.0002"
                copper, then 0.0002-0.0003 brite tin plate outermost layer.

     ~All these pins are then reflowed onto pcb with bare copper pads using
      62Sn/36Pb/2Ag solder paste.

     ~The pins are then pulled vertically and the forces at which the
      solder joints break are recorded. The pins are L-shaped with the lower
      part of the L reflowed on the pcb. We pull the vertical part of the L.

     ~Result :  Average breaking force of Type A > Type B > Type C.
                (some pads are peeled off from the pcb in Type A but the
                solder joints still not break)

                The force of Type B is 32% higher than Type C, but is 8%
                lower than Type A.

     ~Some of the pins (not pulled) are subject to thermal shock at -40 and
      100 degC for 100 cycles. The same pull test is done every 10 cycles.

     ~Result :  Average breaking force order same as above. BUT the forces
                are all INCREASED. For Type A, increased by 6%, for Type B,
                increased by 15%, for Type C, increased by 8%.

                After the shock, the average breaking force of Type A and
                B are almost the same.


     Q1.Why the average solder joint breaking force for tin plated pins
        are lower than that of gold plated pins? I presume that tin
        plated components will solder better.

     Q2.Why thinner gold plated pins have stronger joints than thicker
        gold plated ones?

     Q3.Why there are increases in solder joint breaking force after thermal
        shock? Why the thick gold plating pins have so significant change?

     Q4.I think the solder joints are subject to both tensile and shear
        forces. But is it that solder joints most often break at their
        intermetallic layer(most brittle layer in the joint?)? If yes, is there
        any relationship between brittleness and tensile strength or shearing
        strength?

     Q5.At the solder and pin interface, I think that for tin plated pins, the
        wetting is due to the formation of copper-tin intermetallic, for gold
        plated ones, the intermetallic should be tin-nickel. Which intermetallic
        is stronger relative to pulling and shearing? Which is more brittle.


     Regards,

     Ken Fong
     Process Engineering Section Leader
     Advanced Manufacturing Department
     ACP
     e-mail : [log in to unmask]

################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TechNet <your full name>
To unsubscribe:   SIGNOFF TechNet 
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################