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What is Step-up & Step-down Stencil? Selective Solder Paste Volume Increasing Solution

Step_down_stencilToday, the SMD parts become smaller and smaller. Their sizes can be developed to 0402, 0201, and even 01005 (Note 1). Moreover, the pitch between the pins of a common IC (integrated circuit) is shorten to fine pitch of 0.5mm, even 0.3 mm. The improvement becomes a challenge for the SMT process.

A great challenge for each SMT engineer is how to weld these thin electronic parts on circuit boards without any non-wetting or short-circuit. In addition, it is more difficult to solder few larger parts and a lot of thin ones on a circuit board than to solder thin parts without any larger ones.

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Cooper Defined vs. Solder Mask Defined pad design for BGA soldering strength

Cooper Defined vs. Solder Mask Defined pad design for BGA soldering strength

Since the electronic product design moves from desktop to portable devices, the manufacturer also make electronic component as small as possible to meet this popular trend. Besides, the PCB thickness also become thinner and the associated soldering pads on the PCB are tiny size. This  make the SMT manufacturing process more difficult and challenge.

You may know that portable device always has BGA malfunction risk during impact drop test. Most of company dispense the underfill glue to fix this kind of BGA crack issue. My company did the same thing too.  WorkingBear think there shall be another way to enhance the soldering pad strength to eliminate the underfill glue since underfill process spend money and waste labor hours.

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Why thinner PCB will need carrier for reflow process?


As the career of Manufacturing Process Engineering(MPE), WorkingBear always find there is rope wall between R&D, Marketing and Manufacturer. The R&D and Marketing always like the design be thin, sleek, and modern looking. The manufacturer always like the assembly be robust, solid, rigid, and not easily scratch on the surface. But both of them same ask low cost.

So, here comes two common questions from R&D and marketing people.

  1. Why thinner PCB will ask special carrier or template for the reflow process?
  2. Aren’t all PCBs reflowed as a panel, which requires a support fixture for de-panelization?

Since WorkingBear is MPE, so I will answer these two questions as manufacturing point.

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Case study for the mold-in insert screw boss broken

Case study for the mold-in insert screw boss broken

Last time WorkingBear mentioned the potential root causes to bring the screw boss crack after mold-in screw nut insert. Actually we received the complaint from the field then asked EMS and plastic supplier to double check their inventory and found about 1% of the screw boss cracked after mold-in screw nut insert before box build assembly. That means no extra force and screw driver give to these screw bosses and the crack happen just from its inner stress. The defect rate increase to 30% after screw in the plastic case together.

So, what make this happened? Workingbear had pointed out 4 potential root causes  to bring the screw boss crack for the mold-in nut screw insert process.

  • Using too much of the re-grind resin
  • No pre-heat on the screw Insert parts
  • The improper molding parameter
  • Design risk 

Since we had proved the boss crack really happened from plastic parts itself. We held these poor quality plastic parts and question the plastic supplier why crack happen? The interesting is that plastic supplier challenged me back that they didn’t change any parameter for this plastic parts and they really don’t know what happen and how come the screw boss crack now?

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Selectively increase solder paste volume solutions

Why need to partially increase the solder paste volume?

This is in order to meet current smaller and thinner electric component design, the stencil thickness is made as thin as possible to accurately control the tiny solder paste which print on the PCB.

Unfortunately, there is not only small component be placed on the board but also includes big components. So how can this big electronic SMD component get enough solder paste volume to mount on the board? The stencil thickness is too thin to provide more volume and stencil aperture also can’t be opened  too much over pad size. There shall be another solution to provide extra solder paste volume to this kind of big component and also for the DIP components will apply  PIH (Paste in Hole) process.

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