Ed,
Where does one start?!
HMM, the primary solderability test for years has essentially been dip and look - either manual or supposedly done with mechanical dippers. But the bottom line is one looked at the soldered sample and determined the amount of surface that was in contact with the solder and calculated how much was wetted (covered) by the solder. Now this works well for "giant" dips and the color coded resistors we played with as kids, BUT hand dipping sure doesn't work too well for 0402's and 0201's!!! And as George Wenger has pointed out, for exact dipping nothing works better than a wetting balance!
For leaded devices, chip components and LCC's the way to go today is really to use a wetting balance. I will use an analogy to explain how it works. If you stand on a bathroom scale, you weigh so much. If a small child then comes along and pulls on your pant leg, the scale says you weight more. It is a dumb scale. It cannot tell the difference between your weight and a force applied to you. A wetting balance is nothing but a balance with an internal clock. When the molten solder wets the component, the surface tension/buoyancy (more later) wets/pushes up on the component and the balance part of the wetting balance/solderability tester/meniscograph (spelling?) measures this change with time and display it with force changes on the y axis and time on the x-axis. In NA and Europe the machines available usually show positive wetting forces above the zero line and in Japan it is often the other way around.
The wetting balance curves often have the first part below the zero x line. This is because the sample has displaced the molten solder and Archimedes principle is in effect - the weight of the water displaced.... (of course in this case it is the weight of the molten solder). As the sample heats up and the flux is activated the sample component surface is wetted by the molten solder and the negative meniscus becomes smaller and smaller and finally zero. As the meniscus becomes positive and the component is essentially being pulled down (like the kid pulling on the pant leg) the wetting balance registers a positive force.
Now sometimes the initially wetting is so fast that you will not see the dip below the zero x line. The wetting curve will be almost vertical and then it will usually level off. This is because the wetting force is fighting gravity. The dense solder is sticking to the component lead above the equilibrium surface but gravity is trying to pull it back down. The meniscus will only rise to the point where the wetting force equals gravity. This is all captured in the basic equation for the wetting balance, but I am just too lazy to go into that right now.
Now I said the curve usually levels off. There are several cases where it will not. If the component is a thermal heat sink the curve may have a stair step shape. If the part has crummy, but not terrible wetting, it might wet continually but slowly. And then sometimes it will level off and then slowly decrease. The latter is due to dewetting.
OK, you can look at a curve and an experienced user of the equipment can usually say by looking at the curve whether the component is good or bad. But this is a machine that can produce numbers. What exactly can you get? Well there are a variety - time to reach the buoyancy line, time to two thirds of the maximum wetting force, maximum force, force at time x, % dewetting, area under the curve. I am sure that others can give you a couple more. Suffice to say they are either time or force based.
I am NOT going to go into detail on industry standards here. I will say that the IPC specs are dated (being worked on) and do not list the wetting balance as an "official" method. I haven't got my EU specs in front of me (My office is in another building.) but I can tell you the ones about the wetting balance are pretty detailed. For the life of me I can't remember if they list pass/fail criteria. Personally I use T2/3Fmax equal to or less than 1 second and Force at 2 seconds equal to or greater than 25% of theoretical max and force at five seconds 90% of force at two seconds (to allow for some dewetting).
And finally you have to remember that any type of solderability testing is just a snapshot. It has no statistical validity for a situation where you are fighting a 200 ppm problem. Don't believe me? Get a stats master to work out how many samples you would have to test to be 95% confident that what you are seeing is real!! Of course this is not going to stop me from using the info to beat a supplier over the head if I can. :)
Hope this helps.
regards,
Bev Christian
Research in Motion
-----Original Message-----
From: Edward Szpruch [mailto:[log in to unmask]]
Sent: January 22, 2004 9:57 AM
To: [log in to unmask]
Subject: [TN] basic info on soldering
Hi all,
I am working on different finishes for "lead free" applications .Recently I
got results of solderability tests as graphs showing "wetting force vs time"
and "contact wetting angle vs time" , but I do not know how to interpretate
it.
I would appreciate basic info on this stuff.
thanks
Edward
Edward Szpruch
Eltek Ltd
P.O.Box 159 ; 49101 Petah Tikva Israel
Tel ++972 3 9395050 , Fax ++972 3 9309581
e-mail [log in to unmask]
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
-----------------------------------------------------
|
