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

Step_down_stencilThe trend in the electronics industry is toward smaller and smaller SMD components, with sizes such as 0402, 0201, and even 01005 becoming common (Note 1). Additionally, the pitch between pins on ICs has decreased to a fine pitch of 0.5mm, and even 0.3mm, posing a challenge for the SMT process.

SMT engineers face the challenge of soldering these thin electronic components onto PCBs without encountering non-wetting or soldering short issues. This task is more difficult when dealing with a mix of larger and smaller parts on a circuit board.

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Copper 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 tug of war between Designer, Marketing and Manufacturer. The Marketing always like the design be thin, sleek, and modern looking. The designer always think fabrication is easy. 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

In a previous discussion, WorkingBear identified potential root causes for screw boss cracking after mold-in screw nut insertion. Following a complaint from the field, we asked EMS and the plastic injection supplier to double-check their inventory and found that approximately 1% of the plastic case had cracked on the screw bosses due to inner stress during the mold-in screw nut insert process, without any additional force or screwdriver. However, the defect rate increased to 30% after the screws were fastened into the plastic case.

So, what make the crack 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 

After confirming that the screw boss cracking was caused by the plastic parts themselves, we held these poor quality plastic parts and questioned the plastic injection supplier about why the cracking occurred. Interestingly, the plastic supplier challenged us back, stating that they did not change any injection  parameters for these plastic parts, and they do not know why the screw boss cracking is happening.

This experience has led Workingbear to believe that we are dealing with a very poor plastic injection supplier. I am considering taking matters into my own hands and closing this supplier in the future.

This supplier is particularly interesting because they not only produce the plastic parts but also provide box build assembly and packing services to customers. They perform box build assembly manually, even using manual screwdrivers to screw the top and bottom case together in the production line. Moreover, this supplier was responsible for building the original product. We moved the product out of this supplier two years ago but kept the plastic parts there because many EMS companies do not accept small quantity orders, and that why this supplier can still survive.

It is evident that this supplier has an attitude problem and lacks an engineering and technology background. Their engineers only know how to build parts and products, but not why they should do things that way. Their management is also unwilling to confront problems and is keen on avoiding any trouble as soon as possible.

1. Supplier said they usually do the same way without any crack happened in the past but now happen in other EMS factory?

Case study for the mold-in insert screw boss broken

The supplier claimed that they typically applied a torque of 1.5kgf-cm using a manual screwdriver to secure these Philips M2 machine screws between the top and bottom case. They also challenged the notion that the crack may have been caused by another EMS applying a torque of 2.0~3.0Kgf-cm using a semi-automatic screwdriver. However, Workingbear points out that a good parts even applying a larger screw torque till screw stripped rather than cracking the plastic boss.

I must say that this supplier is lucky not to have received any complaints in the past. However, as time passes, this crack will gradually become more serious. It does not mean that no cracks were found in their original products, but the cracks did not break through to the outside of plastic case.

2. Why needs to do pre-heat the INSERT nut since the plastic injection process tooling will be pre-heated to around 110°C already?

Case study for the mold-in insert screw boss broken

Almost of the INSERT parts are made by cooper material and its coefficient of thermal expansion (CTE) is 16.4 µm/m-°C while stay at 20°C~100°C environmental. The plastic resin is PC and its CTE is 66.6 µm/m-°C while stay at -40°C~+40°C.  This creates a CTE difference of approximately 4 times between the two materials.

Of course, the injection machine can pre-heat the tooling and INSERT nut parts. It is challenging to ensure that the INSERT parts are heated to their desired temperature. Since the only contact between the plastic tooling and INSERT parts is the bolt, there is no guarantee that the pre-heating process will heat the INSERT parts to the right temperature in a short amount of time.

As the high temperature melted resin flows into the tooling cavity, it increases the temperature and size of the INSERT parts. As the resin solidifies and cools down, it shrinks in size while the INSERT parts may still be at a high temperature. This results in a difference in expansion rate between the two materials, leading to inner stress at the interface of the resin and INSERT parts. If the plastic strength is weaker than the inner stress, the weak point may crack or break.

To prevent this from happening, the INSERT parts should be pre-heated to 50°C~60°C and kept at that temperature for at least 10 minutes before being placed into the tooling. This allows the INSERT parts to reach their stable expansion size before the molding process. Pre-heating to 60°C is for safety purposes to prevent the operator from getting burned.

In general, larger screw boss diameters increase the risk of screw boss cracking during the mold-in process. This is because the dimensions of the resin shrink during the cooling stage, causing both the outer and inner diameters to shrink simultaneously. However, the INSERT nut will only shrink slightly and must withstand the stress caused by the resin shrinkage. The greater the diameter of the screw boss, the greater the shrinkage dimension, and if the plastic strength is weaker than the inner stress, then cracking or breakage may occur at the weak point.

So, WorkingBear took below actions to fix this screw boss crack issue

1. Ask plastic supplier to do pre-heating for the INSERT nut at 60°C temperature and keep 10 minutes at least then put it into the injection molding tool.

Actually, the problem was fixed and no crack show after this action. I did use 8Kgf-cm semi-auto screw driver to lock the screw into the INSERT nut for test and didn’t find any crack. I tracked it after one day, one week, two weeks, one month.  

Thanks god the crack be fixed. That means there really something wrong in plastic supplier side but I can’t and hard to prove it. I personal think supplier shall remove the re-grind resin adding too and just won’t tell me the truth.

2. Do tooling modification for the mold-in screw boss to reduce its diameter from 7.16mm to 5.25mm.

Screw_Insert_uut_modificationMy original case show the outer boss diameter is 7.16mm and INSERT nut has outer diameter of 2.38mm. The boss diameter is 7.16/2.38=3.0 times the insert diameter. I was thinking the proposal of 1.5 times from design guideline is really small and would like to do the tool modification step by step gradually and not reduce it to 1.5 times at one time. So I reduce the diameter from 7.16mm to 5.25mm (there still have 5.25/2.38=2.2 times and considering the draft angle) and added 0.3mm radius on all core side corner and remove the pre-heating requirement for these INSERT nut process.

Now 2 years had been passed at least and I never heard any screw boss crack issue for this case.

PS: One year later that I asked to move this molding tool out this supplier. I would like to end the nightmare with this supplier.

Related articles:
Trouble Shooting Guide for Plastic Injection

Selectively increase solder paste volume solutions

Why is it necessary to partially increase the solder paste volume in SMT soldering?

To meet the demands of smaller and thinner electronic component designs, stencils are made as thin as possible to precisely control the tiny solder paste printed on the PCB.

However, this creates a challenge when smaller and larger electronic surface-mounted devices (SMDs) need to be mounted on the same board. The stencil’s thinness limits the amount of solder paste that can be deposited, and extending the stencil aperture over the pad size is still not  yet enough to fill the necessary volume of solder. To provide sufficient solder paste volume for larger components, as well as for HTD components, EMS has implemented the PIH (Paste in Hole) process.

Here WorkingBear will provide some alternative solutions that EMS already did to deliver additional solder paste volume:

1. Solder Paste Manually with Semi-Automatic Dispensers

Manual dispense the solder paste by semi-auto dispenser
In some production lines, extra solder paste is manually added to the board’s land patterns using semi-automatic dispensers. This solution is suitable for situations where precise control of the solder volume is not required, such as for grounding tabs of connectors, smart card leads, and SIM card leads. However, this approach has several drawbacks, such as the need for additional operators on the SMT line, difficulty in controlling the solder paste volume and location, and the risk of human error, such as accidentally touching other components that are already in position.

2. Auto dispenser to add solder paste by machine

Auto solder paste dispenser can easily control the solder paste volume, location and less human error risk. However, disadvantage is that investment is needed to setup extra dispenser machine in the SMT production line.

3. Apply the step-up or step-down stencil

Apply the step-up or step-down stencil The step-up or step-down stencil is made by using the laser solution to remove the certain area thickness in the stencil to partially increase or reduce the solder paste thickness volume for the designed components. However, the amount of thickness to step-up or step-down is limited. It can’t be increased or reduce too much from regular stencil thickness. For example, if uses a 0.10mm thickness of stencil then the certain area usually be increased to 0.127mm thickness only and can’t over 0.15mm. This is because most of component that needs to increase solder paste volume had small components locate on the adjacent area. The solder paste volume will be reduced gradually from certain thicker area of stencil to regular area. It is hard to control the gradually solder volume change. Too much of the solder volume will cause solder short for the small components and fine pitch IC.

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

4. Apply the solder preforms

Apply the solder preforms Apply the solder preforms
The solder preforms is a solid solder paste and form as standard shapes such as squares, rectangles, washers and discs. Typical sizes range from .010″ (.254mm) up to 2″ (50.8mm). Different sizes, as well as customized shape are available, but more expensive.

The tape & Reel packing are also ready for the standard performs size to meet pick and place machine to improve the yield rate and save labor hours. It is usually placed on the printed solder paste during SMT process like a chip capacitor is placed using standard pick and place component libraries and equipment. It just like a solder paste needs go through the high temperature reflow oven then melt and joint the electric component with PCB pad together. It is recommended to use preforms with the solder paste together. Because preform is a solid shape and the paste will fix it on PCB without moving during transit in the SMT equipment path.

Related articles:
How to print solder paste on PC Board and screen printing notices?