Chosen Solution
Hi, I have an iPhone 6, I tried replacing U1700 Tristar and ended up ripping TRISTAR_TO_AP_JTAG_SWCLK while wicking the pads. Is this connection important? I have not tried reinstalling the chip.
TRISTAR_TO_AP_JTAG_SWCLK is used to tell the JTAG interface what frequency (kHz) it should be running at when programming the U1700 chip so that when it is being programmed, the data sent to program the chip isn’t been sent too fast. TL:DR - This pad is not needed as it is used solely for programming the U1700 chip - Programmed already anyway. Source: JTAG_CLK explained - https://community.nxp.com/thread/329958 JTAG_CLK - http://prntscr.com/f45si0
Generally speaking, all of the connections are important. However as @benjamen50 says, this one you can live without. Now this brings up a good point…cleaning up the board after you lift an IC. A lot of the older videos online will show “agressive” use of desoldering braid to get the pads perfectly solder free and flat. This is usually a great recommendation for regular boards but the problem with iDevice pads is that they are so thin and so small. It really doesn’t take much heat or abrasiveness to rip them from the pcb. Many techs today are using concave tips to wick away (via surface tension) the solder. Then you clean the tip, apply fresh lead-free solder and do the same to apply a thin coat. The pads don’t have to be perfectly flat, just consistently flat. This can still lift pads but a lot less frequently than with braid.
