High-frequency board soldering is not a simple scaled-up version of ordinary FR-4 boards. The molecular chains of PTFE-based substrates are extremely vulnerable when heated. Once the reflow peak temperature exceeds 260°C, irreversible collapse of the internal microporous structure of the board will occur, while the adhesion at the copper foil-medium interface rapidly decays. This damage is difficult to identify under AOI, but it will expose fatal defects such as sudden increase in insertion loss or phase inconsistency during RF testing.

Substrate Thermal Characteristics Necessitate Reflow Profile Reconstruction

We have tested Rogers RO4350B and Taconic RF-35 boards on the JJY SMT line, and found that their measured Tg values are only between 155°C and 165°C, far below the 170°C+ of conventional FR-4. This means the substrate enters the softening critical zone starting from 150°C. If the dwell time in the soaking zone exceeds 90 seconds, or the peak temperature exceeds 255°C and is maintained for more than 3 seconds, micro-warping at the copper foil edge can be observed in X-ray cross-sections, which directly degrades impedance continuity in the 5G millimeter-wave band.

Nitrogen Environment Is a Rigid Prerequisite, Not a Nice-to-Have

In ordinary air, oxygen accelerates the carbonization of Flux residues. Carbides attached to the PTFE surface form local hot spots, making the actual contact temperature 8°C to 12°C higher than the set value. After JJY switched to nitrogen protection and reduced the oxygen content to below 100ppm, not only was the peak temperature tolerance increased by 5°C, but the solder joint wetting angle was also narrowed from 32° to 24°. This directly reduces the microvoid rate caused by poor wetting, and microvoids are precisely one of the main causes of high-frequency signal reflection.

Squeegee Pressure and Speed Must Be Calibrated Synchronously

High-frequency board soldering has stringent requirements for Solder Paste printing consistency, because the dielectric thickness tolerance is usually controlled within ±0.025mm. If the squeegee pressure fluctuates by more than ±0.5kg or the speed deviates by more than ±5mm/s, uneven solder volume at the pad edges will result. This unevenness causes local thermal stress concentration during reflow, which in turn induces microcracks in the substrate. We once had to rework a batch of 28GHz radar modules at our Dongguan factory due to this issue; the root cause was that the solder paste thickness CV value deteriorated from 12% to 18.7%, and this fluctuation was entirely caused by pointer jitter on the squeegee pressure gauge.

Furnace Temperature Test Boards Must Be From the Same Batch and Same Stackup as Mass Production Boards

The furnace temperature profile calibrated with standard FR-4 test boards will be systematically high when applied to high-frequency boards. This is because the thermal conductivity of PTFE is only 0.2W/m·K, less than one-third that of FR-4, with a thermal response lag of 4.2 seconds. JJY now mandates that before each batch of high-frequency boards goes online, a dedicated temperature measurement board must be fabricated using PCBs of the same part number, same stackup, and same copper thickness. Otherwise, even if the KIC profile meets the standard, the actual temperature rise inside the real board may still exceed the limit by more than 2.8°C.