Jul01
Why thinner PCB will need carrier for reflow process?

BGA_underfill_crack01BGA_underfill_crack02

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.

1. Why thinner PCB will ask special carrier or template for the reflow process?

PCB_deform_during_reflow_process Before answering these questions that we must understand Current non-RoHS PCB often apply TG150 resin (There are higher TG PCB resin but cost also be higher ) but peak temperature of RoHS reflow is around 240°C~250 °C. That means the PCB will transfer to rubbery state while PCB go into the high temperature zone of reflow oven. Please be noticed that there are many components locate upon the PCB already. The weight of PCB itself and components will give a force of gravity and make PCB become a smile curve. This deformation will be recover a little after reflow and during the cooling stage but it will not return to original perfect flatness before reflow. The thinner and lower TG resin of PCB the more worse deformation. As experience show that if the PCB thickness is less than 1.0mm then the PCB deformation will become serious. WorkingBear will recommend applying 1.6mm thickness of PCB if no space concern.

According IPC-A-610, section 10.2.7 of Laminate Conditions – Blow and Twist, defines the PCB deformation or warpage after solder shall not exceed 1.5% for through-hole and 0.75% for surface mount printed board applications. WorkingBear would like to say the PCB warpage is really critical for the PCB Assembly quality, especially there is BGA package on board. It is easily to bring the Head-In-Pillow (HIP) soldering defect . You can refers to this article of Why Burn/In(B/I) still cannot screen out the DDR soldering fail? first. WorkingBear will find time to introduce the HIP.

There is another concern for the thinner PCB. Please refers to two pictures at this article beginning  show the BGA underfill crack. Yes, the underfill around and under the BGA was cracked. This case that PCB only had 1.0mm thickness but the underfill crack after 120 cm drop test. You may image the drop test make the plastic case deformed then warped the PCB and broke the underfill glue and ball solder for the BGA too.

So, if RD insist on the lower thickness of PCB then how can we do? Applying the reflow carrier or template that made by allium alloy material or composite stone material will be a general idea. There are many of the alignment pins were designed on the Reflow Carrier according to actual NPTH (Non-Plating Hole) location on the panel PCB. The reflow carrier can sustain more higher temperature and can be re-cycle use and It will help to support the PCB and keep the PCB without bending  too seriously.

If reflow carrier is a must jig to support the thinner PCB in the reflow then it will need about 30~50 pcs quantity to keep the SMT production running smoothly. You must calculate how quantity of reflow carrier will be inside the reflow same time, how many carrier will be buffer to move them from  after reflow to before reflow zone. If the PCB thickness is thinner (0.8mm, 0.6mm or FPC)  and them can’t be kept flatness on the solder paste printing or pick & place stage then the carrier quantity will be increased to about 60~80 pcs. Because the carrier will be applied from solder paste printing, pick & place, reflow, AOI process. As experience show the reflow carrier may cost you about US100~US200 per unit.

2. Aren’t all PCBs reflowed as a panel, which requires a support fixture for de-panelization?

Here  we must understand the  SMT process flow,as below .  Please reflow to How EMS fabricate a PCB Assembly? for detail information:

Step 1, Bare board loading into SMT production line

Step 2, Solder paste printing

Step 3, Solder Paste Inspection (Option)

Step 4, Pick and place speed machine

Step 5, Pick and place general machine (low speed)

Step 6, Hand place component  and visual inspection (Option)

Step 7, Reflow

Step 8, Auto Optical Inspection (Option)

Step 9, Unloading PCB Assembly (SMT finessed)

Step 10,  Visual Inspection

Step 11,  Touch-Up (Option)

Step 12, In-Circuit Test

Step 13, Board level Function  Verification Test

Step  14, PCB Assembly  de-panel

So yes, almost of the PCBs reflowed as panel and requires a support fixture for de-panellization but  these two carriers  are different between reflow carrier and de-panelization vacuum carrier. Please be noticed that the  de-panelization vacuum carrier is for router machine to remove the break-away from panel PCB only. There is no de-depanelization carrier  need if PCB design V-cut and apply Scoring machine.

The reflow carrier is designed  to prevent the PCB deformation during reflow and it must sustain cycling high temperature and be survivor.  So its material will be metal alloy or Higher Tg resin.

The de-panelization carrier is designed to hold the PCB in position without moving during milling cutter remove the break-away or connection ribs between board to board. The de-panelization carrier also need to connect to vacuum system to clear the unnecessary break-away material from the panel board.


2 Responses to Why thinner PCB will need carrier for reflow process?

  1. JIMIWHA says:

    If the PCB is 1.0mm thickness, what’s the point of judgment suggestion to use the fixture in SMT or not ?
    Is It still a confidence reliability spec of watching the warpage about 1.5% and 0.75% (user’s filed issue?)
    ( For a NB or 3C product )

    • WorkingBear says:

      Jimi;
      I do think the acceptable criteria for the PCB warpage rate shall base on the actual requirement. Especially for the BGA and MLCC and other bending sensitive components. The IPC standard will be the minimum requirement only. Some products must be survivor after tumbler test and impact test but some desktop products don’t have drop test requirement. It is hard to define the PCB warpage rate for all products.
      The warapged PCB is not only affect to the PCBA reliability but also affect to SMT yield rate. The PCB go 1st reflow will be warpaged then 2nd reflow will be a problem.
      I only can say the 0.75% is general requirement and most of the PCB fabricator follow it. If you ask smaller warpage rate for PCB then the cost may be raised.

Leave a Reply