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Top-Side Preheat and Avoiding Board Warp in Selective Soldering
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mize this in multilayer boards, dis- tribute copper area and weight equally about the theoretical center- line —the “neutral axis.” Bowing and twisting is usually the result of fiber layers not being perpendicular to each other. Most PCBs are based on a laminate of multiple layers of woven glass fibre cloth and epoxy. These laminates have so-called vis- coelastic material properties. This
reinforcements and work to restrict even expansion. The result can be some degree of wave, sag or bow. Also, the weight of components,
shields and heat sinks plays a role in board deformation. If the PCB con- tains heavy heat sinks, the thermal properties of the heat sinks result in cooler areas and limit expansion locally. Physically heavy components or
board features serve to apply weight directly to the weakest areas (typical- ly an unsupported center) and cause sag during the transi- tion phase. Shielding, or any mechanical device that limits movement of the board, can result in uneven expansion and therefore a negative change in coplanarity.
Damage Control The old standard for
An example of a multi-panel board with design features likely to warp during preheat or soldering.
means that properties such as elas- ticity and thermal expansion drasti- cally change above a certain temper- ature, called the “glass transition temperature” (Tg). When soldering the PCB assembly, the laminate will be exposed to temperatures above the glass transition temperature. Even though the epoxy will try to expand in all directions, mechanical devices, such as PTH and vias, act as
warp mitigation is to use a pallet or fixture. The advan- tage is that the board is held on four edges, so it is sup- ported on all sides. The use of pallets also facilitates mech - anical features to support the board in areas of weakness.
The major issues with using pallets are added cost and reduced flexibili- ty in a high-mix environment. The most common machine fea-
ture used to mitigate warpage is side-clamping both in a horizontal and vertical plane. The functionality of such systems is limited due to the fact that the board in most cases is held and clamped on only 3 mm (0.12 in.) of two edges (not four).
PCB deformation profile. To adequately deal with these
factors it is necessary to use a closed- loop real-time detection and correc- tion system. Mapping of the board topography prior to any of the heat cycles yields inconsistent results. With the addition of a few moni-
toring points, the Pil larhouse system and software can measure any change in the board height (X/Y and theta already corrected with fiducials) and adjust accordingly. All it takes is to simply check the box next to any joint that needs correction and the
a more homogenous selective solder- ing process. In addition, many defects in the board, including board warp, twist, sag, and others, can be avoided by carefully controlling and monitoring the process with closed- loop detection and correction sys- tems. Contact: Pillarhouse USA, Inc.,
201 Lively Boulevard, Elk Grove Village, IL 60007 % 847-593-9080 fax: 847-593-9084 E-mail:
sales@pillarhouseusa.com Web:
www.pillarhouse.biz r
Unitech UC-250M-CV Board Cleaner
Winner of the 2016 NPI Award for Cleaning Equipment!
The UC-250M-CV Board Cleaner offers a dual cleaning feature using a combination of a brush roller with the silicone/adhesive cleaning rollers. The combination dual dust removal system assures better results than a single brush or adhesive roller system. The UC-250M-CV has the ability to clean the top surface of PCB even chip components attached on the bottom-side. Additionally, the cleaner features an antistatic bar (ionizer) to neutralize static and prevent dust from collecting on the top surface of the PCB.
TheUC-250M-CVBoardCleaneroffe ersa
email:
info@seikausa.com web:
seikausa.com
26218 Industrial Blvd. Hayward, CA 94545 Tel: 510-293-0580 Fax: 510-293-0940
3528 Torrance Blvd. Suite 100 Torrance, CA 90503 Main: 310-540-7310 Fax: 310-540-7930
1580 Boggs Rd. Suite 900
Duluth, GA 30096 Main: 770-446-3116 Fax: 770-446-3118
The actual shape of the PCB
change is not repeatable in most cases. Predominantly, the board changes occur during preheat cycles, but also as part of the selective sol- dering process due to the localized heating required to solder specific components.
machine automatically connects the warpage point to the joint in the pro- gram library. The system also has the ability to follow complex contours that are completely different in direction and shape from one board to the next. Using techniques such as closed-loop top-side preheat allow for
July, 2016
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