PCB / Stencil and solder paste

Assembly · 12

Stencil and solder paste

Paste volume is set by aperture and foil thickness. QFN and ESP32 modules fail here more often than in the pick-and-place program.

01 What happens

  • Laser-cut stainless stencil, often 0.1–0.12 mm for 0402/QFN, thicker for big power pads.
  • Apertures are reduced vs copper (area ratio). Thermal pads are windowed so they do not void as one lake.
  • SPI (solder paste inspection) checks volume before parts land.

02 On an ESP32 board

  • ESP32-WROOM-class modules need a vendor paste drawing, not a 1:1 copper copy.
  • 0.4 mm pitch: too much paste bridges; too little opens.
  • Through-hole USB that is paste-in-hole needs a different aperture than a chip resistor.

03 What usually fails

  • No paste reduction on QFN — voids and float.
  • Stencil not stepped, so a 0.8 mm connector starves neighbouring 0402.
  • Home-cut Mylar stencil on a fine-pitch first article.

04 What we check

  • We ask for the module vendor’s recommended stencil, then adapt.
  • First article should include SPI or at least a paste visual before reflow.

← Previous step: Electrical test of the bare boardNext step: Pick and place →

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PCB工艺 / 钢网与锡膏

组装 · 12

钢网与锡膏

锡量由开口和钢网厚度决定。QFN 和 ESP32 模组死在这里的，比死在贴片程序上的多。

01 这一步做什么

  • 激光不锈钢网，0402/QFN 常用 0.1–0.12 mm，大功率盘可以更厚。
  • 开口比铜盘缩小（面积比）。散热垫要分窗，避免一摊锡空洞。
  • SPI 在贴片前查锡量。

02 落到 ESP32 板上

  • ESP32-WROOM 一类模组要用原厂锡膏图，不要 1:1 照铜皮开。
  • 0.4 mm：锡多桥连，锡少开焊。
  • USB 通孔蘸锡和电阻开口不是同一套。

03 常见失败

  • QFN 不分锡——空洞、器件浮起。
  • 钢网没有台阶，0.8 mm 连接器旁边的 0402 没锡。
  • 细间距首件用家用薄膜网。

04 我们查什么

  • 先要模组厂推荐钢网，再改。
  • 首件最好有 SPI，至少回流前目检锡膏。

← 上一步: 光板电测下一步: 贴片 →