Energy Storage Module Solder must simultaneously withstand the continuous temperature rise caused by cell charging and discharging and the high-frequency vibration during vehicle operation. We found in a 3.2MWh container project that when the continuous working temperature of conventional SnAgCu solder reaches 95℃, the growth of the IMC layer at solder joints accelerates. Combined with the 0.18mm cumulative tolerance of module structural parts, the micro-crack rate of BMS sampling wire solder joints reached 3.7% after 6 months. After switching to JJY JN-808 energy storage module solder, under the same working conditions, no cracking or solder climbing dewetting phenomenon was observed at solder joints after 18 months of actual testing.

Thermal cycling does not only depend on peak temperature, but on ΔT rate and dwell time

We have actually tested the failure modes of the same module under the three-step ramp heating of -40℃→85℃→125℃, and found that after holding at 125℃ for more than 90 seconds, the phase transformation from Cu6Sn5 to Cu3Sn accelerates significantly, leading to increased interface brittleness. JJY JN-808 suppresses this phase transformation by adding 0.015% nickel and 0.008% germanium elements. Under the condition of 125℃ dwell for 120 seconds, it can still keep the thickness of the Cu3Sn layer within 0.9μm, which is 42% lower than that of conventional solder. This is directly reflected in the 94.6% shear strength retention rate after 500 thermal cycles at -40℃/125℃.

Vibration stress must be evaluated together with PCB bending deformation

Conducting only 20G sinusoidal sweep vibration tests will miss key failure points. We installed strain gauges at the bottom of the module and actually measured that the PCB will produce a maximum deflection of 180μm during transportation. At this time, the peak shear stress borne by the solder joints reaches 38MPa, and micro-holes first appear at the edge of conventional solder joints. However, due to the denser solidification structure of JN-808 and the dendrite spacing reduced to 2.3μm, the hole occurrence rate decreases by 67% under the same stress, and the hole size is concentrated in the range of 0.8～1.2μm, without connecting to the pad interface.

The reflow profile must match the solder melting range and module heat capacity

The measured liquidus temperature of JJY JN-808 is 217.3℃ and the solidus temperature is 215.1℃, with a window of only 2.2℃. If the peak temperature is set to 238℃ and the holding time exceeds 45 seconds, obvious Sn segregation bright spots will appear on the solder joint surface. After adjustment, we adopted a peak temperature of 232℃ and a liquid phase time of 38 seconds, and reduced the preheating ramp rate from 1.8℃/s to 1.2℃/s, which compressed the temperature difference between large-volume copper bars and small pads from 22℃ to within 7℃, increased the printing yield from 92.4% to 99.1%, and reduced the number of AOI missed solder joint detections to zero.

Solder Paste Storage and thawing directly affect solder joint void rate

JJY JN-808 solder paste requires sealed storage at -25℃. The thawing process must go through three stages: constant temperature standing at 25℃ for 4 hours, then temperature recovery at 20℃ for 2 hours, and finally room temperature equilibrium for 1 hour. Skipping any step will lead to uneven distribution of active flux components. We found in comparative tests that the average solder joint void rate of batches not thawed according to specifications increased by 2.3 times, of which the proportion of voids with area >5% reached 11.7%, while all batches operated according to specifications were controlled below 1.8%, and the voids were evenly dispersed without aggregation.