Operation Plating Switch

Silver to Gold

Flexibility, the ability to change, is a valuable asset.  Anyone can tell you it's easier conceived than executed. 

Figure 1: Solder on BGA pads.

Take BGA devices for example. When engineers are developing or upgrading products, the main focus of their manipulations is often a BGA device. In order to fiddle with the device's programming etc. engineers may want to change out the device.  There isn't an easy way to repeatably change out a large-scale BGA device. Too many contacts, too much sensitivity, heck it's hard enough connecting a thousand pins in the first place. Socketing BGA devices is challenging. 

Connecting BGA devices to a BGA board substrate using z-axis compression material is even more challenging.  Soldering the device directly to the board has more than its share of pitfalls.  Sure, all these things are done successfully thousands of times a day. However - pause for a breath - try changing out large devices over and over again and the rate of success drops dramatically.

Well, as we say, if it was easy they wouldn't pay any of us to do what we all do.  

Figure 3: Conductive ink applied on the bottom side of board to complete circuit for plating.

As hard as it is to perform any of the above reliably, there are times when people must actually shift between two different methods. For instance, we commonly get requests to dismantle a socketed or z-axis compression arrangement and place a BGA component directly on a board. Why? Either the socket isn't working as desired or the engineers have settled on the device they want to use and now want to place the component directly on the board and to secure the individual joints with a soldered connection.

As if that isn't crazy enough, we also receive requests to migrate a soldered component into a socket arrangement. The problem with that request is that soldered devices have, well, solder all over the pads. Solder's wear characteristics and the uneven planarity of a soldered surface, cause it to be unusable for BGA sockets.  As a result the soldered surface, for instance, all 1000 pads, require removal of the solder and the new plating of a proper mating surface, usually gold. That, my friends, is a piece of work.  

Figure 2: Tape is used to isolate and provide a fluid disposal pathway.

Let's take a look at a where this solder (what inexperienced people in the industry often refer to as silver because of its appearance) to gold conversion leads us. In Figure 1 there is a soldered surface, the location where a BGA component used to comfortably reside. The customer wants the surface of all these BGA pads to become flat and gold plated.  

That's simple enough, but as we all know, the devil is in the details.  In order to plate one must establish an electrical connection.  How do you make the required electrical connection to all 1000 pads? How do you prevent the stripping and plating chemicals from contaminating the rest of the board? Of course we only want to plate gold on the desired pads, how do we keep it off of other surfaces?  

Figure 4: Finished and gold plated pads.

Figures 2 and 3 indicate how these things might be accomplished. Thankfully, in this instance the BGA pads are connected to plated through hole vias and these vias run through to the other side of the board.  If a common electrical connection can be made to all these vias then a complete circuit can be established to the BGA pads on the top side for plating. A disolvable conductive ink can be applied to the underside of the board and cured to connect all of the vias.  The same ink is placed on top of a buss wire which is clipped to the cathode of the plating unit. Sweet! Additionally kapton tape and non-permanent mask is used to isolate the plating surface from the surrounding board and to provide a pathway for the excess plating solutions to be washed away from the area.

Once the time consuming preparatory steps are completed and all the pads metered for connection, the plating process begins.  Actually, this step is the easiest. The preparation and clean-up are the things that really take time.  Once the gold plating is laid down, the surface is inspected. If the inspectors are kind, the conductive ink and kapton tape are removed and the component location is ready for a socket.

Figure 4 shows what a properly plated pad looks like.  Easy as 123 and 456789......

Several members of the Circuit Technology Center team contributed to this feature story.